Albert Barnes Commentary 1 John 5:17

"All unrighteousness is sin: and there is a sin not unto death." — 1 John 5:17 (ASV)

All unrighteousness is sin, and so on. This statement seems to be inserted to qualify what he had just said. And there is one great and enormous sin, a sin which could not be forgiven. But he also says that there are many other forms and degrees of sin, sin for which prayer may be offered.

Everything, he says, that is unrighteous—adikia—everything that does not conform to the holy law of God, and that is not right in the view of that law, is to be regarded as sin; but we are not to suppose that all sin of that kind is of such a nature that it cannot possibly be forgiven.

There are many who commit sin who, we may hope, will be recovered, and for them it is proper to pray. Deeply affected as we may be, considering that there is a sin that can never be pardoned, and much as we may pity one who has been guilty of such a sin, yet we should not hastily conclude in any case that it has been committed, and should constantly keep in mind that while there is one such sin, there are multitudes of other sins that may be pardoned, and that for those who have committed these pardonable sins, it is our duty to pray unceasingly.