If we say we have no sin, we delude ourselves and the truth is not in us.1 John 1:8
Back in verse 6 John begins a rapid fire set of “if/thens.” Verses 8-10 continue that rhetorical tack. John is setting sail in the glorious sea of Gospel conquest. The first question that arises in setting out on that voyage is obvious: how should the one who is now in fellowship with God understand their relationship with sin (αμαρτια)? Are they still under sin’s dominion if they still commit sinful actions?
John is interested in proclaiming the Gospel which deals with sin. The Gospel doesn’t turn a blind eye to sin, neither does it redefine sin so as to get us off the hook. Two tendencies frequently arise in church history, either legalistic self-righteousness or permissive self-indulgence. One insists that everything is a sin, the other that nothing is. Legalism tries to overcome indwelling sin by looking to one’s own righteousness, and thus must redefine their sin in clever ways, they call it ”holy indignation” instead of an outburst of wrath, or a ”mistake in judgement” rather than an act of selfishness.
The libertine denies there is such a thing as sin at all, clearing the way to indulge in every vice. along with thinly veiled allusions to God’s overwhelming love and kindness. One spits in the face of Christ’s righteousness, the other spits in the face of God’s justice. Both are fundamental misunderstandings of the saint’s relationship to sin.
Though we are delivered from the power of sin and the penalty of it through Christ’s precious blood, yet there remains indwelling sin which must be resisted and overcome. Later on, John will flesh out how the saint overcomes the world (1 Jn. 5:4). What is being established here in the beginning, however, is that denying the presence of remaining corruption in the believer’s life is self-delusion. It is like poking holes in the bottom of the ship and wondering why it ended up in the bottom of the harbor. The Westminster Confession of Faith is profoundly eloquent on this point: “In which war [with sin], although the remaining corruption for a time may much prevail, yet, through the continual supply of strength from the sanctifying Spirit of Christ, the regenerate part doth overcome: and so the saints grow in grace, perfecting holiness in the fear of God (Chapter 13, Sec. III).”
All this to say, denying your sin while professing to be a believer is to prove that you are not a believer. Truth doesn’t dwell in you. The saint acknowledges their sin (the very next verse, 1 Jn. 1:9), and so proves that they are a child of the True God. God’s people are those who bring their sin into the Light of God’s truth.