Summary: We need the cleansing that God offers.
Psalm 51 (quickview)  is one of the few psalms where we are given the historical background. The inscription reads, "A Psalm of David when Nathan the prophet came to him, after he had gone in to Bathsheba." That identifies clearly for us the incident out of which this psalm arose.
It was the time when David became involved in the double sin of adultery and murder while he was king. He had walked with God for many years. He had gained a reputation as a prophet, a man who understood the deep things of God; and he had established himself as the long time spiritual leader of his people. Then suddenly, toward the end of his reign, he became involved in this terrible sin.
The interesting thing is that David himself records this sin for us. It must have been a painfully humiliating experience for the king. You remember the story. He was on his palace roof one day when the army had gone out to battle and he saw a beautiful woman next door bathing herself. His passion was aroused and he sent over messengers and ordered her to be brought to him. He entered into an adulterous relationship with her because she was a married woman. Her husband, a soldier in David’s army, was away fighting for his king.
Later, when David learned that she was expecting a child, he panicked and tried to cover up his actions. He ordered the husband, Uriah, to be sent home from battle, hoping that he would sleep with his wife and the child would then be accepted as his own. But Uriah was a faithful soldier, committed to battle, and though he came home at the king’s orders, he would not go into his own house but slept with the soldiers at the palace and returned to the battle the next day.
David knew that ultimately his sin would be found out so he took another step. That’s always what sin does -- it leads us on deeper and deeper, farther than we ever intended to go. Before the king knew it, he found himself forced into a desperate attempt to cover up his evil. He ordered Uriah, the husband, to be put in the forefront of the battle where he would most certainly be killed. And when news of Uriah’s death reached King David he felt he was off the hook, he had safely covered his sin. But his conscience continued to bother him.
In Psalm 32 (quickview)  David records how he felt during that terrible time when he was trying to cover up his sin. He said, “When I kept things to myself, I felt weak deep inside me. I moaned all day long.” (Psalm 32:3 (quickview) , NCV). For about a year, he tried to live with a guilty conscience.
Do you remember Edgar Allen Poe’s story, “The Telltale Heart”? In that story, the main character has committed murder and he buries the body of the victim in his basement. But the murderer is unable to escape the haunting guilt of his deed. He begins to hear the heartbeat of his dead victim. A cold sweat pours over him as that heartbeat goes on and on, relentlessly, getting louder and louder. Eventually, it becomes clear that the pounding which drove the man mad was not in the grave below but in his own chest.