Word study erusin and nisuin

Word Study — אירוסין (erusin) and נישואין (nisu'in)

Jewish marriage in the Second Temple period happened in two distinct legal stages, not one ceremony. Matthew, writing in Greek, has his own vocabulary for marking these stages — worth laying alongside the Hebrew terms.

Erusin (אירוסין) — "betrothal/engagement."

Despite the English word "engagement," this stage was already a full legal marriage in terms of obligation. It was usually formed by money or a written contract; intercourse could also create the bond, but was not the expected first step. Once erusin was formed:

• The woman was legally the man's wife — Scripture and rabbinic law call her his ishah (wife), not merely his intended.
• Breaking it required a formal divorce (get), not just a change of mind.
• Unfaithfulness during this period was treated legally as adultery, not merely as breach of promise — which is exactly why Joseph's option in v.19 is framed as divorce, not "calling it off."
• If the man died during this period, she was a widow, not a jilted fiancée.

Greek term: μνηστευθείσης (mnēsteutheisēs, from μνηστεύω, "to betroth/engage") — the participle used of Mary in v.18. This is not modern "dating" or a casual understanding; μνηστεύω names the same legally-binding stage as erusin. Greek has no separate word for a casual engagement, because the underlying institution wasn't casual.

What erusin did not include yet: cohabitation or physical union. The couple remained in their separate households, often about a year.

Nisu'in (נישואין) — "marriage" proper, sometimes called chuppah (named for the bridal canopy).

This was the second stage: the groom brought the bride into his home, the marriage was consummated, and full married life began. This is the stage still future for Joseph and Mary in Matthew 1:18.

Greek term: συνελθεῖν (synelthein, from συνέρχομαι, "to come together") — the infinitive in the phrase "before they came together" (πρὶν ἢ συνελθεῖν αὐτούς). Here it means the union of life and body — the couple becoming one household — not merely "meeting up." This is Matthew's way here of marking that nisu'in has not yet begun.

So the Greek phrase in v.18 is doing precise, technical work: μνηστευθείσης marks that erusin is already in force, and πρὶν ἢ συνελθεῖν marks that nisu'in has not yet begun. The pregnancy is discovered in the gap between them.