He gives signs for those who are willing to take the chance. Matthew is telling us that Isaiah's prophecy, whether he knew it or not, is fulfilled in the conception of Jesus. The incarnation of God's Son is a sign to the world that God has kept his promises, just as he always does. Joseph was to find comfort in the fact that Mary hadn't been unfaithful to him, but that God had been faithful to his people by coming among them through Jesus. Amen