1 John 1:7

"but if we walk in the Light as He Himself is in the Light, we have fellowship with one another, and the blood of Jesus His Son cleanses us from all sin."

Hebrews 10:10 & 14

"And by that will, we have been made holy through the sacrifice of the body of Jesus Christ once for all.... because by one sacrifice he has made perfect for ever those who are being made holy."

If Christ died for only part of our sins, then God would have had to send His Son a second time to die. But since Christ died for all our sins, He will never have to be sacrificed again!