But it was, as the prophets also predicted, from a virgin that there was to be born, according to the promised sign, one who was to give His name to the fact, showing that at His birth God was to be with man. Now it seems to me appropriate to the character of a Jew to have quoted the prophecy of Isaiah, which says that Immanuel was to be born of a virgin. This, however, Celsus, who professes to know everything, has not done, either from ignorance or from an unwillingness (if he had read it and voluntarily passed it by in silence) to furnish an argument which might defeat his purpose.
And the prediction runs thus: “And the Lord spoke again unto Ahaz, saying, Ask you a sign of the Lord your God; ask it either in the depth or in the height above. But Ahaz said, I will not ask, neither will I tempt the Lord. And he said, Hear now, O house of David; is it a small thing for you to weary men, but will you weary my God also? Therefore the Lord Himself shall give you a sign. Behold, a virgin shall conceive, and bear a son, and shall call His name Immanuel, which is, being interpreted, God with us.” And that it was from intentional malice that Celsus did not quote this prophecy, is clear to me from this, that although he makes numerous quotations from the Gospel according to Matthew, as of the star that appeared at the birth of Christ, and other miraculous occurrences, he has made no mention at all of this.
Now, if a Jew should split words, and say that the words are not, “Lo, a virgin,” but, “Lo, a young woman,” we reply that the word “Olmah”— which the Septuagint have rendered by “a virgin,” and others by “a young woman”— occurs, as they say, in Deuteronomy, as applied to a “virgin,” in the following connection: “If a damsel that is a virgin be betrothed unto an husband, and a man find her in the city, and lie with her; then you shall bring them both out unto the gate of that city, and you shall stone them with stones that they die; the damsel, because she cried not, being in the city; and the man, because he humbled his neighbour's wife.” And again: “But if a man find a betrothed damsel in a field, and the man force her, and lie with her: then the man only that lay with her shall die: but unto the damsel you shall do nothing; there is in her no sin worthy of death.”
Source: Against Celsus (New Advent)