But David , NOah or Moses did not Sin every day, did they? It was very rare that they sinned once they knew the Lord.

This is what I'm saying that a Christian can sin in his life. However, the sin should not appear every day. If the sin appears every day, then we are in an addiction of some sort. If we are in an addiction, then we need to pray and ask God for freedom from the addiction.

What I'm trying to say, is that we should not sin every single day. If we do sin, then what are different from an unsaved person?

What I'm saying that a Christian can sin once in a while, because the Christian did was not on guard. But the Christian should not sin regularly. Also, the Christian try his best to not sin.