At the very moment of the Fall, God addresses the serpent: 'I will put enmities between thee and the woman, and thy seed and her seed: she shall crush thy head, and thou shalt lie in wait for her heel' (Genesis 3:15). The Fathers called this verse the Protoevangelium — the 'first Gospel' — because before a single page of salvation history unfolds, victory over the serpent is already promised through the offspring of a woman.
The Church reads the fulfillment as beginning at Nazareth, where the New Eve gave her consent and the Word took flesh of the woman (Luke 1:26–38; Galatians 4:4 — 'God sent his Son, made of a woman'), and as completed at Calvary, where the serpent's head was crushed. St. Irenaeus (c. 180) built his whole theology of recapitulation on this verse.
A textual note shown for both sides: the Latin Vulgate reads 'ipsa conteret' — 'SHE shall crush thy head' — which shaped centuries of Marian iconography (Our Lady's foot upon the serpent). The Hebrew reads 'he/it shall crush,' referring to the woman's seed; modern Catholic translations follow the Hebrew. The doctrine is unaffected: the victory belongs to the seed, Christ, in which His Mother uniquely shares.