John Piper; 1 John 1:9

Confession and Forgiveness

Verse 9 we have already discussed in connection with verse 7. But let's review its meaning. We can see it best when we put verses 6–7 and verses 8–9 in parallel columns to show their similarity.
Verses 6–7
Verses 8–9
If we say that If we say that
we have fellowship with him and we walk in darkness we do not have sin
we lie we deceive ourselves
and we do not do the truth. and the truth is not in us.
If we walk in the light as he is in the light If we confess our sins
we have fellowship with one another
he is faithful and just to forgive us our sins
and the blood of Jesus his Son cleanses us from all sin and to cleanse us from all unrighteousness.

One of the many insights we get from these parallels is that denying our sin is part of what it means to walk in darkness, and confessing our sin is part of what it means to walk in the light. Which implies further that denying our sinfulness cuts us off from fellowship with God (just the opposite of what we are prone to feel); and confessing our sin opens the channel of forgiveness and cleansing.
Some people say that a Christian should never pray for forgiveness because his sins are all forgiven in Christ. It is finished in the cross and no more forgiveness is possible. There is truth in this. But to me it smacks of artificiality. At least we should say, "Father, I have sinned and am not worthy to be called your child. Please apply the blood of Christ to this my sin and count it among those which you forgave when he died for me on the cross."
But if that is too complex, I am sure the Father would gladly receive the words, "Forgive us our sins as we forgive those who sin against us."
Calling God a Liar
Finally, verse 10 repeats verse 8 with a stronger warning: "If we say we have not sinned, we make him a liar, and his word is not in us." The claim to sinlessness is not only self-deception, but blasphemy. It amounts to calling God a liar.
This means that God's assessment of us is not positive. He calls us evil. If we deny our evil, we call him a liar. John uses strong words so we will be strong people.
Let these simple, straightforward, weighty truths sink into your mind and you will have a great ballast to keep your boat from capsizing in the winds of contemporary fads and fashions and trends.