I will sing of the Lord's unfailing love forever! You and old will hear of your faithfulness. Your unfailing love will last forever. Your faithfulness is as enduring as the heavens. The Lord said, "I have made a covenant with David, my chosen servant. I have sworn this oath to him. I will establish your descendents as kings forever; they will sit on your throne from now until eternity".
God's unfailing love, it is a really amazing thing. God has made a covenant with Abraham and eventually with David. A covenant is usually a two-way street. Each member of the covenant has a responsibility to the other party. When God says to the people I will be your God, and you will be my people. There is an expectation on the people that they remain faithful to God. They struggled with faithfulness from the very beginning. They doubted God at the Red sea, and worshipped a golden calf at Sinai. God remained always faithful. They didn't trust God enough to enter the land, and so God had them wandering in the wilderness until an entire generation had died off. When they did enter the land, they never followed God fully. They were to drive out all the inhabitants of the land, but they allow many to remain in the land. God still remained faithful. When they demanded a King like other men, God allowed it. God continued to remain faithful to them. They followed after pagan idols and lived wickedly, and God remained faithful. He made a covenant with David that his descendents would sit forever on the throne. By the time of Jesus birth this may have seemed like a broken promise by God. Jesus arrives as a descendent of David. He is born in a manger, but he has royal blood. He will sit forever on the throne, because he is God Everlasting. This promise to David is the promise that we celebrate fulfilled in Christmas. Jesus is not only the Savior of the world, but he is the son of David, and born a king to sit on David's throne forever.