After stating the blood of Jesus cleanses the believer from all unrighteousness, he twice declares anyone who denies they have sin is a liar (1:8, 10). Why would anyone claim to “not sin”? It is possible someone thinks they are completely righteous, they actually do not sin. This might be the result of a defective view of sin (only literally breaking the Ten Commandments counts, not our thoughts).
Another possibility is someone thought the behaviors which are sin for some people are not sins for them. Paul deals with a similar problem in Romans 14-15, for some eating meat (presumably sacrificed to idols) was a sin, for others it was not a sin. The church at Corinth struggled with eating food sacrificed to idols, although the situation was complex.
Is this a description of the opponents, those who have left the church? For many commentators n 1 John this is a hint of the beliefs of John’s opponents. Although this is teased this out over the whole book, the “ones who have gone out” from John’s community seem to claim they do not sin. At this point it is impossible to know if this means they do not sin ever, or some particular practice John’s group claims is sin is not really a sin at all. By way of analogy: some Christians think drinking alcohol is a sin, others do not.
But this analogy does not do justice to the problem because John uses very strong language to describe this person: they are a liar and God’s word is not them. That God’s word is not in them may refer to John’s own eyewitness report of the Gospel and the revelation from God he has given to his churches. This type of person is self-deluded and never understood what John had taught them in the first place. Perhaps they read passages like John 3:20-21 and misinterpreted it, thinking John meant if they were in the light whatever they did no longer counted as sin.
“God’s word” can also refer to Jesus in John’s Gospel. If the ones who went out from John’s community claim to have no sin, then they do not have Jesus! This is a self-deception, the one who denies they sin (for whatever reason) knows they do in fact sin and are suppressing that truth so they can continue in that sin. They are telling themselves lies.
John therefore stresses the importance of confessing our sin (1:9). The opposite of denying we sin is to confess our sin (Jobes, 1, 2, & 3 John, 71). This verse can be problematic for several reasons. If we have been declared righteous (as in Pauline Theology, good reformed theology), then God has already completely forgiven all our sin, past present and future. Why should we continue to confess our sin? Does this verse actually command Body of Christ believers to confess their sin in order to be forgiven?
Although he gives no details on how this ought to be done, John does not have in mind here anything at all like the Catholic sacrament of confession. For many modern readers, their only conception of “confess sin” comes through their experience in a Catholic church. Even if it was based on this text, Catholic practice developed over centuries and cannot be what John had in mind.
Equally problematic are unfortunate attempts by evangelical Christian churches to require someone caught in a spectacular sin to make some sort of public confession of their sin to the whole community in order to stay in fellowship with that church. There are many sides to that discussion (theological, practical and legal), but we need to set aside that modern church practice in order to understand John’s point here.
The Jewish practice of confession of sin is an important for understanding this verse. Just as purification is required for fellowship with God on the Temple Mount, so too confession of sin is an important element of worship in the Old Testament.
If we confess our sins we can be assured God will forgive us because he is faithful and just. God’s faithfulness and justice and foundational elements of Old Testament theology, both are part of God’s self-revelation in Exodus 34:6-7. God is faithful to his promises, in this case he has promised forgiveness as part of the new covenant. Since Jesus’s blood has enacted the New Covenant, God promises to forgive our sin.