Albert Barnes Commentary 1 John 1:9

"If we confess our sins, he is faithful and righteous to forgive us our sins, and to cleanse us from all unrighteousness." — 1 John 1:9 (ASV)

If we confess our sins. Pardon, in the Scriptures, always presupposes that there is confession, and there is no promise that it will be imparted unless a full acknowledgment has been made. Compare Psalm 51; Psalms 52; Luke 15:18; Luke 7:41; Proverbs 28:13.

He is faithful. To his promises. He will do what he has assured us he will do in remitting them.

And just to forgive us our sins. The word just here cannot be used in a strict and proper sense, since the forgiveness of sins is never an act of justice, but is an act of mercy. If it were an act of justice, it could be demanded or enforced, and that is the same as saying that it is not forgiveness, for in that case there could have been no sin to be pardoned. But the word just is often used in a larger sense, as denoting upright, equitable, acting properly in the circumstances of the case, etc. Compare Matthew 1:19.

Here the word may be used in one of the following senses:

1. Either as referring to his general excellence of character, or his disposition to do what is proper; that is, he is one who will act in every way as becomes God; or,
2. That he will be just in the sense that he will be true to his promises; or that, since he has promised to pardon sinners, he will be found faithfully to adhere to those engagements; or perhaps,
3. That he will be just to his Son in the covenant of redemption, since, now that an atonement has been made by him, and a way has been opened through his sufferings by which God can consistently pardon, and with a view and an understanding that he might and would pardon, it would be an act of injustice to him if he did not pardon those who believe on him.

Viewed in any of these aspects, we may have the fullest assurance that God is ready to pardon us if we exercise true repentance and faith. No one can come to God without finding him ready to do all that is appropriate for a God to do in pardoning transgressors; no one who will not, in fact, receive forgiveness if he repents, and believes, and makes confession; no one who will not find that God is just to his Son in the covenant of redemption, in pardoning and saving all who put their trust in the merits of his sacrifice.

And to cleanse us from all unrighteousness. By forgiving all that is past, treating us as if we were righteous, and ultimately by removing all the stains of guilt from the soul.