14 Therefore the Lord himself shall give you a sign. Behold a virgin shall conceive, and bear a son and his name shall be called Emmanuel.
Douay–Rheims · Challoner revision · Isaias 7:14
Why this matters
Isaiah gives a sign in the setting of King Ahaz. Matthew 1:22–23 explicitly reads the virgin birth of Jesus as its fulfillment. The Hebrew term and its historical referent are discussed across traditions; the Marian application is mediated through Matthew.