Creation fell under judgement. A curse descended on it. Pain, frustration, sickness, bondage and death have been the order of things ever since. Satan, the dark intruder, works to inject as much harm, chaos and pain as possible. Between them is war.
Earth is the theater of operations.
Christmas is the turning point in the war. Jesus Christ, son of God, laid aside His heavenly glory and became one with the human race. He came to live a perfect life and walk the road to Hell so that we wouldn't have to. He came to face Satan right in the middle of the chaos and pain, and beat this evil tyrant at his own game. For those of you with a sense of Bible history, you may notice that in Jesus' flight to Egypt, his time there, and eventual return - in that He was retracing the history of Israel.