All unrighteousness This passage may be explained variously. If you take it adversatively, the sense would not be unsuitable, "Though all unrighteousness is sin, yet every sin is not unto death." And equally suitable is another meaning, "As sin is every unrighteousness, hence it follows that every sin is not unto death." Some take all unrighteousness for complete unrighteousness, as though the Apostle had said, that the sin of which he spoke was the summit of unrighteousness. I, however, am more disposed to embrace the first or the second explanation; and as the result is nearly the same, I leave it to the judgment of readers to determine which of the two is the more appropriate.

All unrighteousness is sin - This seems to be thrown in to guard what he had just said, and there is "one" great and enormous sin, a sin which could not be forgiven. But he says also that there are many other forms and degrees of sin, sin for which prayer may be made. Everything, he says, which is unrighteous - ἀδικία adikia - everything which does not conform to the holy law of God, and which 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 character 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 in view of the fact that there is a sin which 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 bear constantly in mind that while there is one such sin, there are multitudes that may be pardoned, and that for them it is our duty unceasingly to pray.

All unrighteousness is sin - Πασα αδικια, Every act contrary to justice is sin - is a transgression of the law which condemns all injustice.

(16) All unrighteousness is sin: and there is a sin not unto death.
(16) The taking away of an objection: indeed all iniquity is comprehended under the name of sin: but yet we must not despair therefore, because every sin is not deadly, and without hope of remedy.

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.

"Every unrighteousness (even that of believers, compare 1-John 1:9; 1-John 3:4. Every coming short of right) is sin"; (but) not every sin is the sin unto death.
and there is a sin not unto death--in the case of which, therefore, believers may intercede. Death and life stand in correlative opposition (1-John 5:11-13). The sin unto death must be one tending "towards" (so the Greek), and so resulting in, death. ALFORD makes it to be an appreciable ACT of sin, namely, the denying Jesus to be the Christ, the Son of God (in contrast to confess this truth, 1-John 5:1, 1-John 5:5), 1-John 2:19, 1-John 2:22; 1-John 4:2-3; 1-John 5:10. Such wilful deniers of Christ are not to be received into one's house, or wished "God speed." Still, I think with BENGEL, not merely the act, but also the state of apostasy accompanying the act, is included--a "state of soul in which faith, love, and hope, in short, the new life, is extinguished. The chief commandment is faith and love. Therefore, the chief sin is that by which faith and love are destroyed. In the former case is life; in the latter, death. As long as it is not evident (see on 1-John 5:16, on 'see') that it is a sin unto death, it is lawful to pray. But when it is deliberate rejection of grace, and the man puts from him life thereby, how can others procure for him life?" Contrast James 5:14-18. Compare Matthew 12:31-32 as to the wilful rejection of Christ, and resistance to the Holy Ghost's plain testimony to Him as the divine Messiah. Jesus, on the cross, pleaded only for those who KNEW NOT what they were doing in crucifying Him, not for those wilfully resisting grace and knowledge. If we pray for the impenitent, it must be with humble reference of the matter to God's will, not with the intercessory request which we should offer for a brother when erring.

All deviation from perfect holiness is sin; but all sin is not unpardonable.