John Piper writes on 1 John 1:8,
The claim to be without sin is self deception. And the reason we are so deceived is that the truth has found no place in us. When a person is converted, the truth takes up residence in the heart. Or we could say the light of God comes into the heart. And what John says here is not that it immediately drives out all falsehood and sin. That is a battle that lasts a lifetime. What he says is that when the truth enters in, its light reveals sin! The mark of the saint is not sinlessness but sin-consciousness! The evidence of indwelling truth is the exposure of error. The dawning of God’s light in the heart is the revelation of remaining darkness.
In this life we never get beyond the awareness of remaining sin. Therefore one of the great signs of maturity in Christ is a deep and abiding brokenness for sin.
There is much talk today about esteeming ourselves as new creatures in Christ. And so we are. But our newness consists in this: that the true light is shining in our hearts revealing the dreadfulness of our remaining sin and the abundance of God’s grace. Our great joy is that our sin is forgiven in Christ. And our great grief is that so much of this very sin remains and defiles.
The mark of the new creature in Christ is not a rosy self-concept. It is brokenness for remaining sin mingled with a joyful confidence in the superabounding grace of God in Christ.
Excerpted from Let Us Walk in the Light of God.