1Jo 1:8 If we say that we have no sin, we deceive ourselves, and the truth is not in us.
1Jo 1:9 If we confess our sins, he is faithful and just to forgive us [our] sins, and to cleanse us from all unrighteousness.
Here, I. The apostle, having supposed that even those of this heavenly communion have yet their sin, proceeds here to justify that supposition, and this he does by showing the dreadful consequences of denying it, and that in two particulars: - 1. If we say, We have no sin, we deceive ourselves, and the truth is not in us, 1Jo_1:8. We must beware of deceiving ourselves in denying or excusing our sins. The more we see them the more we shall esteem and value the remedy. If we deny them, the truth is not in us, either the truth that is contrary to such denial (we lie in denying our sin), or the truth of religion, is not in us
The denial of our sin not only deceives ourselves, but reflects dishonour upon God. It challenges his veracity. He has abundantly testified of, and testified against, the sin of the world.
But God has given his testimony to the continued sin and sinfulness of the world, by providing a sufficient effectual sacrifice for sin, that will be needed in all ages, and to the continued sinfulness of believers themselves by requiring them continually to confess their sins, and apply themselves by faith to the blood of that sacrifice. And therefore, if we say either that we have not sinned or do not yet sin, the word of God is not in us, neither in our minds, as to the acquaintance we should have with it, nor in our hearts, as to the practical influence it should have upon us.
The apostle then instructs the believer in the way to the continued pardon of his sin. Here we have, 1. His duty in order thereto: If we confess our sins, 1Jo_1:9. Penitent confession and acknowledgment of sin are the believer's business, and the means of his deliverance from his guilt. And, 2. His encouragement thereto, and assurance of the happy issue. This is the veracity, righteousness, and clemency of God, to whom he makes such confession: He is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness
God is faithful to his covenant and word, wherein he has promised forgiveness to penitent believing confessors. Henry