If we claim to be without sin,
we deceive ourselves
and the truth is not in us.
Now, most people, when asked, would say that they are sinners. Most people wouldn't claim to be without sin. They know that they mess up all of the time.
But what does their lifestyle say?
It is so easy to know the "theological" answer, but ignore that truth in our lives. So we can say that we are sinners, but we lack all of the signs of a person who truly believes that they are currently with sin. There is no Godly sorrow, no repentance, and no desire to clear oneself of these things (2 Cor. 7:11).
So they claim sinlessness by their actions.
If you claim sinlessness, you are truly deceived. Everyday should be a day of seeking to be right with God by repenting of our sins.