Whatever is not of faith is sin
"When the Lord said of the Holy Spirit, 'He shall convict the world of sin,' he meant unbelief. For this is what he meant when he said, 'Of sin because they believed not on me.' And he means the same when he says, 'If I had not come and spoken to them, they should not have sin.' (John 15:22). He was not talking about [a time] before they had no sin. Rather, he wanted to indicate that very lack of faith by which they did not believe him even when he was present to them and speaking to them. These were the people who belonged to 'the prince of the power of the air, who now works in the children of unbelief' (Ephesians 2:2). Therefore those in whom there is no faith are the children of the devil because they have nothing in their inner being that would cause them to be forgiven for whatever is committed either by human infirmity, ignorance or any evil will whatever. But the children of God are those who certainly, if they should 'say that they have no sin, deceive themselves, and the truth is not in them,' but immediately (as it continues) 'when they confess their sins' (which the children of the devil do not do, or do not do according to the faith which is peculiar to the children of God), 'he is faithful and just to forgive them their sins and to cleanse them from all unrighteousness'" (1 John 1:9).