Christmas is a sign and a proof that the promise of Jesus' return will be fulfilled. He came once and just as he came once he will come again! God promised to send the world a Savior and God sent us His only begotten Son. God had not given up on the human race then, and has not given up on us yet.
Dr. Eugene Boring explains the inspiration of Scripture by comparing it to atomic radiation. When an atomic bomb goes off, we describe the area hardest hit as "Ground Zero." Dr. Boring says that Jesus was Ground Zero of God's breaking into our world. And the Bible is inspired partly because of the nearness to the blast of Jesus' presence.
Regarding Jesus' Second Coming, remember that one day is with the Lord as a thousand years, and a thousand years as one day. The Lord is not slow concerning his promise; but is patient with us, not wishing that any should perish
We did not follow cunningly devised fables, when we made known to you the power and coming of our Lord Jesus Christ, but we were eyewitnesses of his majesty.
The Bible teaches us that God's time is different from what we experience. Think about the way things grow. A seed is placed in the ground and then there is a period of waiting. When the time is right a bit of green will pop out of the soil and, even then, the plant must have time to grow and develop into a tasty vegetable or a lovely flower. Like a plant that takes time to grow, we learn that "The Lord… is patient with you" (each one of us). God gives us time to accept his love and understand our faith, to really know what we believe.
We call this a first hand account – something you see and hear and experience. When you actually see something happen you believe it! You know what you saw and heard and believe to be true. The disciples give a first hand account of seeing the majesty of Jesus and hearing God's voice say: "This is my Son, my Beloved, with whom I am well pleased."