But that we may not seem, because of a Hebrew word, to endeavour to persuade those who are unable to determine whether they ought to believe it or not, that the prophet spoke of this man being born of a virgin, because at his birth these words, “God with us,” were uttered, let us make good our point from the words themselves. The Lord is related to have spoken to Ahaz thus: “Ask a sign for yourself from the Lord your God, either in the depth or height above;” and afterwards the sign is given, “Behold, a virgin shall conceive, and bear a son.” What kind of sign, then, would that have been— a young woman who was not a virgin giving birth to a child?
And which of the two is the more appropriate as the mother of Immanuel (i.e., “God with us”)—whether a woman who has had intercourse with a man, and who has conceived after the manner of women, or one who is still a pure and holy virgin? Surely it is appropriate only to the latter to produce a being at whose birth it is said, “God with us.” And should he be so captious as to say that it is to Ahaz that the command is addressed, “Ask for yourself a sign from the Lord your God,” we shall ask in return, who in the times of Ahaz bore a son at whose birth the expression is made use of, “Immanuel,” i.e., “God with us?”
And if no one can be found, then manifestly what was said to Ahaz was said to the house of David, because it is written that the Saviour was born of the house of David according to the flesh; and this sign is said to be “in the depth or in the height,” since “He that descended is the same also that ascended up far above all heavens, that He might fill all things.” And these arguments I employ as against a Jew who believes in prophecy. Let Celsus now tell me, or any of those who think with him, with what meaning the prophet utters either these statements about the future, or the others which are contained in the prophecies?
Is it with any foresight of the future or not? If with a foresight of the future, then the prophets were divinely inspired; if with no foresight of the future, let him explain the meaning of one who speaks thus boldly regarding the future, and who is an object of admiration among the Jews because of his prophetic powers.
Source: Against Celsus (New Advent)