Origen of Alexandria, Against Celsus 1.35
We do not want to appear to offer comfort based on a Hebrew text to readers who cannot follow it, leaving them to decide whether to accept it or not. So let us take the very wording of the prophecy and find our reassurance there: the prophet said that this one would be born of a virgin, and it is of his birth that the words "God with us" are spoken (Isaiah 7:14; Matthew 1:23). Scripture records that the Lord said to Ahaz, "Ask for yourself a sign from the Lord your God, into depth or into height" (Isaiah 7:11), and the sign that follows is, "Behold, the virgin will conceive and bear a son" (Isaiah 7:14). What sort of sign would it be for a young woman who is not a virgin to give birth? And to whom does it more fittingly belong to bear the name "Emmanuel" — that is, "God with us" — to a woman who has had intercourse and conceived through sexual passion, or to one who is still pure, holy, and a virgin? For it is fitting that such a woman should bear the child at whose birth the words "God with us" are declared. If someone tries to argue cleverly that the command "Ask for yourself a sign from the Lord your God" was addressed specifically to Ahaz, we will answer: in the time of Ahaz, who was born to whom the words "Emmanuel, which is God with us" apply at his birth? If no such person can be found, then clearly what was said to Ahaz was spoken to the house of David, because the Savior is written to have come "according to the flesh" from the seed of David. Moreover, this "sign" is described as reaching "into depth or into height," since "he who descended is also he who ascended above all the heavens, that he might fill all things" (Ephesians 4:10). I say these things as one addressing a Jew who accepts the prophecy. But let Celsus, or anyone who sides with him, tell us: with what kind of mind does the prophet speak about future things — these or any others recorded in the prophecies? Does he possess a mind that knows the future, or not? If he does, then the prophets had a divine spirit. If he does not, let him explain the mind of the man who dares to speak about the future and is admired by the Jews for his prophecy.
Greek
Ἵνα δὲ μὴ δοκῶμεν ἀπὸ λέξεως Ἑβραϊκῆς τοῖς μὴ καταλαμβάνουσι, πότερον συγκαταθετέον αὐτῇ ἢ μὴ, φέρειν παραμυθίαν περὶ τοῦ προφήτην εἰρηκέναι ἐκ παρθένου τεχθήσεσθαι τοῦτον, ἐφ᾿ οὗ τῇ γενέσει λέλεκται τὸ „μεθ᾿ ἡμῶν ὁ θεὸς“, φέρε ἐπ᾿ αὐτῆς τῆς λέξεως παραμυθησώμεθα τὸ λεγόμενον. ὁ μὲν κύριος ἀναγέγραπται εἰρηκέναι τῷ Ἄχαζ· „αἴτησαι σεαυτῷ σημεῖον παρὰ κυρίου τοῦ θεοῦ σου εἰς βάθος ἢ εἰς ὕψος,“ ἑξῆς δὲ τὸ διδόμενον σημεῖον τὸ „ἰδοὺ ἡ παρθένος ἐν γαστρὶ ἕξει καὶ τέξεται υἱόν.“ ποῖον οὖν σημεῖον τὸ νεάνιδα, μὴ παρθένον τεκεῖν; καὶ τίνι μᾶλλον ἁρμόζει γεννῆσαι „Ἐμμανουὴλ,“ τουτέστι „μεθ᾿ ἡμῶν ὁ θεὸς,“ ἆρα γυναικὶ συνουσιασθείσῃ καὶ διὰ πάθους γυναικείου συλλαβούσῃ ἢ ἔτι καθαρᾷ καὶ ἁγνῇ καὶ παρθένῳ; ταύτῃ γὰρ πρέπει γεννᾶν γέννημα, ἐφ᾿ ᾧ τεχθέντι λέγεται τὸ „μεθ᾿ ἡμῶν ὁ θεός.“ ἐὰν δὲ καὶ οὕτως εὑρεσιλογῇ λέγων ὅτι τῷ Ἄχαζ εἴρηται· „αἴτησαι σεαυτῷ σημεῖον παρὰ κυρίου τοῦ θεοῦ σου,“ ἀπαντήσομεν· κατὰ τοὺς χρόνους τοῦ Ἄχαζ τίς ἐγεννήθη, ἐφ᾿ οἷ τῇ γενέσει λέγεται τὸ „Ἐμμανουὴλ, ὅ ἐστι μεθ᾿ ἡμῶν ὁ θεός,·“ εἰ γὰρ οὐδεὶς εὑρεθήσεται, δηλονότι τὸ τῷ Ἄχαζ εἰρημένον τῷ οἴκῳ εἴρηται Δαυὶδ διὰ τὸ „ἐκ σπέρματος Δαυὶδ“ ἀναγεγράφθαι τὸν σωτῆρα γεγονέναι „τὸ κατὰ σάρκα“· ἀλλὰ καὶ τὸ „σημεῖον“ τοῦτο „εἰς βάθος ἢ εἰς ὕψος“ λέγεται εἶναι, ἐπεὶ „ὁ καταβὰς οὗτός ἐστι καὶ ὁ ἀναβὰς ὑπεράνω πάντων τῶν οὐρανῶν, ἵνα πληρώσῃ τὰ πάντα.“ ταῦτα δὲ λέγω ὡς πρὸς τὸν Ἰουδαῖον συγκατατιθέμενον τῇ προφητείᾳ. λεγέτω δὲ καὶ Κέλσος ἤ τις τῶν σὺν αὐτῷ, ποίῳ νῷ ὁ προφήτης περὶ μελλόντων ἢ ταῦτα ἢ ἕτερα λέγει, ὁπόσα ἀναγέγραπται ἐν ταῖς προφητείαις· ἆρα γὰρ προγνωστικῷ μελλόντων ἢ οὔ; εἰ μὲν γὰρ προγνωστικῷ μελλόντων, θεῖον εἶχον πνεῦμα οἱ προφῆται· εἰ δ᾿ οὐ προγνωστικῷ μελλόντων, παραστησάτω τὸν νοῦν τοῦ ἀποτολμῶντος καὶ λέγοντος περὶ μελλόντων καὶ θαυμαζομένου παρὰ τοῖς Ἰουδαίοις ἐπὶ προφητείᾳ.