1 John 5:17

All unrighteousness is sin
All unrighteousness against God or man is a sin against the law of God, and the wrath of God is revealed against it, and it is deserving of death; yet all unrighteousness is not unto death, as the sins of David, which were unrighteousness both to God and man, and yet they were put away, and he died not; Peter sinned very foully, and did great injustice to his dear Lord, and yet his sin was not unto death; he had repentance unto life given him, and a fresh application of pardoning grace:

and there is a sin not unto death;
this is added for the relief of weak believers, who hearing of a sin unto death, not to be prayed for, might fear that theirs were of that kind, whereas none of them are; for though they are guilty of many unrighteousnesses, yet God is merciful to them and forgives, ( Hebrews 8:12 ) , and so they are not unto death.