The Virgin Birth — Isaiah 7:14 and Matthew 1

Matt 1:22-23 reads the birth of Jesus as the fulfillment of Isa 7:14: "the virgin shall conceive and bear a son, and they shall call his name Immanuel." Matthew hears in Immanuel, "God with us," the whole meaning of the incarnation.

Irenaeus — Against Heresies. Thomas Aquinas — Summa Theologiae. The sign given to faithless Ahaz becomes the sign to the world.

The Immanuel theme frames the whole Gospel, returning in Matt 28:20, "I am with you always." It connects to the Word made flesh in John 1:14 and to the protoevangelium's promised seed in Gen 3:15.