A.B. Simpson emphasizes the significance of the Incarnation as God's ultimate sign to humanity, rooted in the prophecy of Isaiah regarding the virgin birth of Jesus Christ. He illustrates how King Ahaz's refusal to seek God's sign led to dire consequences, while God's promise of 'Immanuel' reveals His deep commitment to humanity. Simpson argues that the miraculous nature of Christ's birth and His life exemplify God's love and the supernatural essence of Christianity, challenging the rationalistic views of the world. He further explains that the Incarnation not only signifies God's presence with us but also serves as a transformative power within believers, urging them to embody Christ in their lives. Ultimately, the Incarnation is a pledge of Christ's return in glory, reminding us of His intimate connection with humanity.