When Matthew opens his Gospel by telling his readers that the birth of Jesus fulfilled what the prophet had spoken — the virgin will conceive and give birth to a son — he is quoting Isaiah 7:14. But he isn’t quoting it from the Hebrew. He’s quoting the Greek translation, the Septuagint, that Greek-speaking Jews and the earliest Christians used as their working Bible. And the Greek word standing at the center of his argument is παρθένος (parthenos).

Parthenos means virgin in the specific sense: a young woman who has not had sexual relations. That precision is exactly what gives Matthew’s prophecy its force. The Hebrew word Isaiah actually used, almah, names a young woman of marriageable age and doesn’t specify virginity. The Septuagint translators, working in Alexandria roughly two centuries before the birth of Jesus, chose parthenos to render it. They had no Christian agenda in view; they may simply have assumed that an unmarried young woman of marriageable age would be a virgin, or read Isaiah’s promised sign as something more dramatic than an ordinary pregnancy. Whatever their reasoning, the choice locked the verse into the specifically-virgin reading long before anyone connected it to Mary.

That is why this single Greek word carries so much weight. When Matthew identifies Isaiah’s parthenos with the mother of Jesus, the whole virgin-birth-as-fulfilled-prophecy argument rests on it. Read from the Hebrew, Isaiah 7:14 gives a young woman of marriageable age conceiving a son — which fits Mary, but doesn’t single out a virginal conception. Read from the Greek, Isaiah is heard specifying, seven hundred years in advance, that the Messiah would be born of a virgin. The Christological weight that has carried two thousand years of Christmas sermons and Marian devotion is real. It is also riding, very specifically, on the word the Alexandrian translators chose.

This is not a case of the New Testament getting something wrong. It is a case of the New Testament writers quoting the Bible they and their audiences actually had in hand. The Greek Old Testament was what was in the air; the apostles breathed it, and their first hearers heard Isaiah’s promise in Greek, in the precise key the word parthenos set. For the modern reader who finds young woman in her Old Testament and virgin in Matthew, the gap is not a mystery to solve but a translation to overhear — the moment where the Greek did the work.