Isaiah 7:14 promises a sign. A young woman, Hebrew almah, is with child and will bear a son called Immanuel. Almah denotes a young woman of marriageable age without specifying virginity one way or the other; Biblical Hebrew has a separate word, betulah, for that. When Jewish translators in Alexandria rendered Isaiah into Greek for the Septuagint centuries before Christianity existed, they chose parthenos, a Greek word that usually does mean virgin, for reasons scholars still debate rather than as a christological move.
That translation choice became a live weapon once Matthew's Gospel quoted the Greek text to support a virgin birth. Justin Martyr, arguing with the Jewish teacher Trypho around the middle of the second century, pressed parthenos as proof and accused contemporary Jewish translators of tampering with the text once Christian use of it became inconvenient. Trypho, in the dialogue as Justin records it, held that almah simply meant young woman and that the whole sign concerned Isaiah's own time, a child born to signal the fall of two threatening kings within the boy's infancy, not a birth seven centuries later.
Jerome inherited the argument when he translated the Hebrew Bible into Latin at the end of the fourth century. He kept virgo, virgin, in his Vulgate rendering of Isaiah 7:14 and defended the choice explicitly against Jewish critics of his day, cementing parthenos and virgo as the Christian reading for over a millennium. Modern critical scholarship, working from the Hebrew rather than the Greek or Latin, has generally returned to Trypho's older point. Almah is not betulah, and the verse's own context names a sign within Isaiah's own political moment.