What does Psalm 51:5 mean?

David acknowledges that he is not only a sinner by practice but also by nature. Like all human beings, David was born with a sinful nature. Paul told the church at Rome: "Therefore, just as sin came into the world through one man [Adam], and death through sin, and so death spread to all men because all sinned" (Romans 5:12). David was not suggesting that sexual intimacy in marriage is wrong, he was simply stating that his human nature was corrupt.

Just as an apple tree bears apples because by nature it is an apple tree, even so all human beings commit sins because they are sinners by nature. Romans 3:23 pronounces "all have sinned and fall short of the glory of God." Because all human beings are sinful by nature, choice, and practice, they cannot enter God's kingdom without receiving a new nature. Jesus told Nicodemus, "You must be born again" (John 3:7). The apostle Peter referred to the new birth as our becoming "partakers of the divine nature" (2 Peter 1:4).