It is essential to notice that the statement means that those who read it do not only need understanding but also faith; and not only faith but also understanding. Those of the circumcision who do not believe in the Christ of God, though even now they hear these words, do not have understanding of the subject of this prophecy because they do not hear with the mind. The only reason for their lack of understanding is their lack of faith, as the prophecy clearly reveals both about them and to them.

That the way to understand, therefore, may be open to you, you do rightly first of all, in professing that you believe; for no one embarks upon the sea and trusts himself to the deep and liquid element unless he first believes it possible that he will have a safe voyage. Neither does the husbandman commit his seed to the furrows and scatter his grain on the earth, but in the belief that the showers will come, together with the sun’s warmth, through whose fostering influence, aided by favoring winds, the earth will produce and multiply and ripen its fruits. Nothing in life can be transacted if there be not first a readiness to believe. What wonder then, if, coming to God, we first of all profess that we believe, seeing that, without this, not even common life can be lived.

According to the teaching of the Catholic church, the Christian mind must first be nourished in simple faith, in order that it may become capable of understanding things heavenly and eternal. Thus it is said by the prophet: Unless you believe, you shall not understand. Simple faith is that by which, before we attain to the height of the knowledge of the love of Christ so that we may be filled with all the fullness of God, we believe that the dispensation of Christ’s humiliation was not without reason, in which he was born and suffered as man, foretold so long before by the prophets through a prophetic race, a prophetic people, a prophetic kingdom.

Unless you believe, you shall not understand, showing that as righteousness is by faith, understanding comes by wisdom. Accordingly, in the case of those who eagerly demand evident truth, we must not condemn the desire but regulate it, so that beginning with faith it may proceed to the desired end through good works.

If you are not able to understand, believe, that you may understand. Faith goes before; understanding follows after; since the prophet says, Unless you believe, you shall not understand.

The mysteries and secrets of the kingdom of God first seek for believing people, that they may make them understanding. For faith is understanding’s step, and understanding faith’s attainment. This the prophet expressly says to all who prematurely and in undue order look for understanding and neglect faith.

So, then, in some points that bear on the doctrine of salvation, which we are not yet able to grasp by reason—but we shall be able to sometime—let faith precede reason, and let the heart be cleansed by faith so as to receive and bear the great light of reason; this is indeed reasonable. Therefore the prophet said with reason: If you will not believe, you will not understand; thereby he undoubtedly made a distinction between these two things and advised us to believe first so as to be able to understand whatever we believe. It is, then, a reasonable requirement that faith precede reason, for, if this requirement is not reasonable, then it is contrary to reason, which God forbid. But, if it is reasonable that faith precede a certain great reason that cannot yet be grasped, there is no doubt that, however slight the reason which proves this, it does precede faith.

Certainly one must have faith in the Scriptures as containing the divine mind, and thus one must proceed to the understanding of what is written in them. For one must go beyond the types and thus apprehend the truth of what has been shown to us. First one must believe in the Scriptures with the simple faith that they are inspired by God and useful^[1] and then go on to examine subtly and enquiringly the meaning contained in them.

And Ahaz said, ‘I will not ask and I will not tempt the Lord.’ It is not from humility but from pride that he does not wish to ask for a sign from the Lord. For although it is written in Deuteronomy, You shall not tempt the Lord your God,^[1] and the Savior would use this as testimony against the devil,^[2] when Ahaz was told to ask for a sign he should have fulfilled the commandment in obedience, especially since both Gideon and Manoah sought and received signs.^[3] Although it was according to the ambiguity of the Hebrew expression ulo enasse adonai that everyone translated this as I will not tempt the Lord, it can also be read as I will not exalt the Lord. For the impious king knew that if he had asked for a sign, he would have received one, and the Lord would have been glorified. Like a worshiper of idols, therefore, who sets up altars on all the street corners^[4] and on mountains and in forests,^[5] he also was a fanatic for capriciousness. He did not want to ask for a sign because he was commanded to do so.

By no means will God speak in many and various ways, according to the apostle Paul,^[1] nor according to another prophet will he be represented through the hands of the prophets,^[2] but he who previously spoke through others will himself say Here I am.^[3] The bride in the Song of Songs also asked in this regard: O that you would kiss me with the kisses of your mouth!^[4] For the Lord of hosts is himself the King of glory.^[5] He will descend to a virginal womb and will enter and exit through the eastern gate that always remains closed,^[6] concerning which Gabriel said to the virgin: The Holy Spirit will come upon you, and the power of the Most High will overshadow you; therefore the one who will be born to you is holy and will be called the Son of God.^[7] And Proverbs writes, Wisdom built itself a home.^[8] Thus when it is said, The Lord himself will give you a sign, this should refer to something new and marvelous.

And again hear what was prophesied through Isaiah the prophet, that he would be born of a virgin. He said, Behold, the virgin shall conceive and bear a son, and they will call his name, God with us. Through the prophetic spirit God announced beforehand that things which are unimaginable and believed to be impossible for human beings would take place, in order that when it occurred it would be believed and received by faith because it had been promised. In order to ensure that someone does not accuse us of saying the same things as the poets, who say that Zeus came to women for sexual pleasure, we will explain the words of this prophecy clearly.^[1] The phrase behold, the virgin shall conceive means that the virgin would conceive without intercourse. If she had in fact had intercourse with someone, she would not have been a virgin. God’s power came on the virgin, overshadowed her and caused her to conceive while she remained a virgin.

Isaiah tells of the mystery of our faith and hope: Behold, a virgin shall conceive, and bear a son, and shall call his name Emmanuel. I know that the Jews are accustomed to meet us with the objection that in Hebrew the word almah does not mean a virgin but a young woman. And, to speak truth, a virgin is properly called bethulah, but a young woman, or a girl, is not almah but naarah! What then is the meaning of almah? A hidden virgin, that is, not merely virgin, but a virgin and something more, because not every virgin is hidden, shut off from the occasional sight of men.^[1]

The Jews say that it is not written in the prophecy virgin but young woman. To which it may be answered that young woman and virgin mean the same thing in Scripture, for in Scripture young woman refers to one who is still a virgin. Furthermore, if it was not a virgin that gave birth, how would it be a sign, something extraordinary? Listen to Isaiah, who says, For this reason the Lord himself shall give you a sign, and immediately he adds, Behold, the virgin. So if it were not a virgin that would give birth, it would not be a sign. The Jews, then, alter the text of Scripture in their malice, putting young woman instead of virgin. But whether the text reads young woman or virgin, it should be understood in either case that it is a virgin who will give birth so that the event may be a miraculous sign.

What is the sign? Behold, a virgin shall conceive and bear a son. In fact, a virgin did conceive and gave birth to Emmanuel,^[1] God with us. This is the new birth: a man born from God. God was born in the man, taking the flesh of the old human race without the help of the old human seed. God took the flesh in order to reform the old human race with a new seed. In other words, he spiritually cleansed the old human race by removing its old stains.

He [God] was with us on the earth, when he assumed flesh; and he was no less God in man, and man in God. That he was both God and man was declared before by the prophets.

The manner of his birth proves the truth about the Lord: a virgin conceived without knowing a man; her belly was filled, having been touched by no embrace; and her chaste womb received the Holy Spirit, whom her pure members preserved and her unsullied body carried. Behold the miracle of the mother of the Lord! She is a virgin when she conceives, a virgin when she brings forth, a virgin after birth. What glorious virginity! What splendid fruitfulness! The world’s goodness is born, and there is no pain of childbirth. The womb is emptied, a child is brought forth, and still virginity is not violated. For it was fitting that when God was born, the value of chastity should increase, and that one who was untouched should not be violated by his coming—he who came to heal what was injured—and that bodily purity should not be harmed by him who bestows virginity on those who have been baptized and had formerly been unchaste. The child who has been born, then, is placed in a crib. This is God’s first dwelling place, and the ruler of heaven does not disdain these straitened circumstances—he whose home was the virginal womb. Clearly Mary was a fit habitation for Christ not because of the nature of her body but because of the grace of her virginity.

But what does this passage mean, if not that God has come in the flesh?

Humanity was blended with God, and he was one. The more powerful predominated in order that I might become god just as he became human. Although he was already begotten, he was born of a woman, who was a virgin. Because his birth was from a woman, it was human. Because she was a virgin, it was divine. He had neither a human father nor a divine mother.

You have heard, therefore, that the Father is called Lord. Come now, and let me show you that the Son is called God. Behold, the virgin shall be with child, and shall give birth to a son, and they shall call his name Immanuel, which means, ‘God is with us.’ Did you see how both the name Lord is given to the Father and the name God is given to the Son? In the psalm, the sacred writer said, Let them know that Lord is your name.^[1] Here Isaiah says, They shall call his name Immanuel.

To prevent you from thinking that his coming to earth was an accommodation, as those others were, and to give you solid grounds for truly believing that his was real flesh, he was conceived, born and nurtured.^[1] That his birth might be made manifest and become common knowledge, he was laid in a manger,^[2] not in some small room but in a lodging place before a throng of people. This was the reason for the swaddling clothes and also for the prophecies spoken long before. The prophecies showed not only that he was going to be a man but also that he would be conceived, born and nurtured as any child would be. Isaiah proclaimed this when he said, Behold, the virgin will conceive and bear a son, and they will call his name Immanuel. He eats butter and honey. And again, the same prophet said, A child is born to us, a son is given to us.^[3] Do you see how these prophecies foretold his infancy?

What precedes this passage also gives us its meaning. He does not simply say, Behold, the virgin will conceive. First he said, Behold, the Lord will give you a sign, and then he adds to it, Behold, the virgin will conceive. If the one who was to give birth was not a virgin but the conception occurred in the natural manner, then what sort of sign would this be? A sign must be extraordinary and strange, or how else could it be a sign?

Christ was born a visible man of a human virgin mother, but he was a hidden God because God was his Father. So the prophet had foretold: Behold, the virgin shall be with child and shall bring forth a son; and they shall call his name Emmanuel, which is interpreted, God with us.^[1] To prove that he was God, Christ worked many miracles, some of which—as many as seemed necessary to establish his claim—are recorded in the Gospels. Of these miracles the very first was the marvelous manner of his birth; the very last, his ascension into heaven in his body risen from the dead.

But who shall say that Christ and Christians have no connection with Israel, seeing that Israel was the grandson of Abraham, to whom first, as afterwards to his son Isaac, and then to his grandson Israel himself, that promise was given, which I have already mentioned, namely: In your seed shall all nations be blessed?^[1] That prediction we see now in its fulfillment in Christ. For it was of this line that the Virgin was born, concerning whom a prophet of the people of Israel and of the God of Israel sang in these terms: Behold, a virgin shall conceive, and bear a son; and they shall call his name Emmanuel. For by interpretation, Emmanuel means God with us.^[2] This God of Israel . . . has forbidden the worship of other gods, . . . has forbidden the making of idols, . . . has commanded their destruction, . . . has by his prophet predicted that the Gentiles from the ends of the earth would say, Surely our fathers have worshiped lying idols, in which there is no profit. This same God is he who, by the name of Christ and by the faith of Christians, has ordered, promised and exhibited the overthrow of all these superstitions.

You must appreciate, brothers and sisters, what a tremendous desire possessed the saints of old to see the Christ. They knew he was going to come, and all those who were living devout and blameless lives would say, Oh, if only that birth may find me still here! Oh, if only I may see with my own eyes what I believe from God’s Scriptures! The saints who knew from the holy Scripture that a virgin was going to give birth as you heard when Isaiah was read: Behold, a virgin shall conceive in the womb and shall bear a son, and his name shall be called Emmanuel. What Emmanuel means the Gospel declares to us, saying, which is interpreted, God with us.^[1] So do not let it surprise you, unbelieving soul, whoever you are, do not let it strike you as impossible that a virgin should give birth, and in giving birth remain a virgin. Realize that it was God who was born, and you will not be surprised at a virgin giving birth. So then, to prove to you how the saints and just men and women of old longed to see what was granted to this old man Simeon,^[2] our Lord Jesus Christ said, when speaking to his disciples, Many just men and prophets have wished to see what you see and have not seen it; and to hear what you hear and have not heard it.^[3]

Do you hesitate or refuse to believe his birth of a virgin, when you ought rather to believe that thus it was fitting for God to be born man? Learn that this, too, was foretold by the prophet: Behold, a virgin shall conceive and bear a son, and his name shall be called Emmanuel, which is interpreted, God with us. You will not doubt, therefore, the motherhood of a virgin if you want to believe the nativity of a God who does not relinquish the government of the universe and comes in flesh among human beings; who bestows fecundity on his mother yet does not diminish her integrity.

The Savior’s name, because of which he is called God with us by the prophet, signifies both natures of his one person. For he who, born before time from the Father, is God himself in the fullness of time, became Emmanuel (that is, God with us) in his mother’s womb, because he deigned to take the weakness of our nature into the unity of his person when the Word was made flesh and dwelt among us.^[1] In a wonderful manner he began to be what we are, while not ceasing to be what he had been, assuming our nature in such a way that he himself would not lose what he had been.

Were she not to be a virgin, the birth would not have been a sign. A sign is something that differs from the normal way things happen, that is outside the natural manner. A sign is so unusual and unexpected that someone who sees it or hears of it sees that it is out of the ordinary. It is called a sign because it is significant. Were the birth to be like normal births, it would not have been significant. If the prophecy is about a woman giving birth in the normal manner, like what happens every day, then why call it a sign?

House of David, from this time on when you encounter your enemies, call upon the one who is named Immanuel. The meaning of the title, which is God with us, reveals the power of the Word. Believe this sign, be courageous, and do not call upon the gods of Damascus. Do not enlist those who have no assistance to offer. Rather, call upon Immanuel, the God who will be with people at the appropriate time. Take heart with confidence, trusting in the power of the title. . . . If the prophet had said, They will call his name Immanuel, he would have been speaking only of a future time. This would have caused doubt for some, because when the Savior was born of the virgin his name was not Immanuel but Jesus, as the angel had instructed Joseph, saying, Do not fear to take Mary as your wife, for that which is conceived in her is of the Holy Spirit. She will bear a son, and you shall call his name Jesus, for he will save his people from their sins.^[1] If the prophecy was they will call his name Immanuel, then how would the Savior had fulfilled it, when his name was Jesus and not Immanuel? But this is not how it was written, because not everyone would call him by this title. The prophetic word says accurately, You shall call. . . . Some scribes, because they did not understand this, wrote, they will call rather than you shall call in the Gospel of Matthew, even though the prophecy does not read this way. The Hebrew word translated in the passage is you shall call, as is used by all the translations. Some translators translate the word for virgin as young woman. There is no reason to think that the virgin was not also a young woman; in fact it is likely that the virgin who conceived the Savior was not fully grown but a young maiden.

Now, since the Lord was not a mere man but was also God and knew all things, he stood in no need of reflection, inquiry, counsel or judgment. He also had a natural affinity for good and antipathy for evil. Thus it is in this sense that the prophet Isaiah, too, says, Before the child shall know to refuse the evil, he will choose the good. For before the child knows to refuse the evil and to choose the good, he will reject the evil by choosing the good. The before shows that he made no inquiry or investigation in a human manner but that since he was God and divinely subsisted in the flesh—that is to say, was personally united to the flesh—by the fact of his very being and his knowing all things he naturally possessed the good.

This is said [The child grew in wisdom and in age before God and men]^[1] in order to establish the truth of his human body. Nevertheless, wrapped in swaddling clothes and fed with curds and honey, he will have the judgment to distinguish between good and evil, that rejecting evil he might choose the good. It does not say that he will in fact reject and choose but that he would learn to reject and to choose, so that we might know through such words that this pertains to the infant’s human body, not to divine wisdom. Finally, it must be believed that the angels who announced to shepherds the news of the infant lying in a manger and the magi who came from the east to worship him were chosen. Herod, the scribes and the Pharisees, on the other hand, were condemned because they slaughtered thousands of children for the sake of one infant.

Do not marvel at the novelty of the thing, if a Virgin gives birth to God.