In Isaiah 7, King Ahaz is fearful of an alliance that is forming against him. To reassure him, the Lord sends Isaiah out to meet him and tell him that the alliance will not be successful. The Lord tells Ahaz to ask for a sign that the Lord has spoken the truth, Ahaz curiously decides not to. Thus, the Lord provides his own sign, which signals the truth of his promise to Ahaz but also testifies about a future moment, when a virgin will give birth to a son who will be named Immanuel.
This sign is fulfilled in the birth of Jesus. Matthew 1:18-25 picks up this prophecy and, in naming Jesus as “Immanuel,” demonstrates how God's plan has been fulfilled. The promised “son” had arrived, and “he will save his people from their sins.” God's deliverance is found in his sign to his people, a baby who grew up to become the Savior of the world.