If we claim that we have not sinned, we make Him a liar and His word is not in us.
There may not be many today who walk around speaking of their sinlessness, but it is all too common for modern man to think he is "good enough" to get into heaven. Most people suppose they will make the cut through their own merit - after all they aren't as bad as other people they know.
But God has throughout history made known the sinfulness of man. As the Psalmist penned, "Surely I was sinful at birth, sinful from the time my mother conceived me." As James wrote, "For whoever keeps the whole law and yet stumbles in one point, he has become guilty of all." If anyone claims to be sinless, or "good enough", they are in essence calling God a liar. Therefore God's word is no way in them. The whole system of Christianity is based on the fact that man is a fallen being in need of a Savior. If any person denies this of course he is not a true Christian.