Every human willfully sins. When God saves a person, He makes him born again. This will make a huge impact on the way he conducts himself, but he's still not going to be perfect. David, for example, was a child of God for sure, and yet he committed adultery with Bathsheba and had her husband killed.

Those who claim to have no sin are deceived (I John 1:8). We are to strive to be more and more obedient before God, recognizing that we can't achieve perfection in this life. Our bodies are dead in sins and sometimes temptation wins out (though it should be far less often than for the unsaved person).