1 John 3:6

Whosoever abideth in him sinneth not: whosoever sinneth hath not seen him, neither knoweth him.

From what John had already stated in 1 John 1, we know that he had no intention here of contradicting himself with any teaching to the effect any one having committed sin was in no sense a Christian. Many of the scholars assure us, based upon the Greek verbs used here, that "sinneth" in this context means "leads a life of sin."

Abideth in him ... This is the key to the sinlessness of Christians, since their sins are forgiven continually through the power of the blood of Christ (1 John 1:7). It is only in such a sense as this that any child of God was ever sinless.