Advent is the time of the year we celebrate the coming of Christ to the world in the Incarnation. This is the gospel of God, that is God's gospel of good news to the world. However, the origin of this promise is not found in the New Testament, but in the Old Testament. The first gospel promise in Genesis 3:14 begins with a curse. The man, the woman, and the serpent are all cursed to live in a fallen world. But God immediately brings the plan of salvation, the good news for man when God says that the seed of the woman will crush the head of the serpent, though the seed of the serpent will bruise His heel. Through the centuries prior to this first Advent of Christ, the prophets spoke of this promise, most specifically in Isaiah that "Unto us a child is born, unto us a son is given". This prophecy is fulfilled when the angel of the Lord on the fateful night of the Incarnation said, "Unto you a Savior is born, to you today the one who was promised is given".
This is what separates our faith in Christian belief and thought from any other religion in the world, that God would redeem us back from a fallen world to a Kingdom prepared for those who believe in this promise, Jesus. Now we wait once again for the final return of our Christ in the second Advent that has yet to come. Come Lord Jesus, come quickly!