Was the man in 1 Corinthians 5 guilty of the sin unto death?
Yes, the man in 1 Corinthians 5 was guilty of a sin unto death. The context identifies his ongoing relationship with his stepmother as worthy of death if he did not repent. If he continued, God would take him out of Corinth. Verse 5 delivers him to Satan for the destruction of the flesh so his spirit may be saved.
Les teaches that the earlier statement about a sin not unto death implies some sin is gross enough that God will take that believer out ahead of time. He then points to 1 Corinthians 5 as the case in point. Les explains that this man was evidently committing a sin unto death—worthy of death if he did not repent and turn around. If he continued living with his stepmother in a marital relationship, God was going to take him out of Corinth. Verse 5 says to deliver such a one unto Satan for the destruction of the flesh, that the spirit, the soul, may be saved in the day of the Lord Jesus. Les then turns to the Corinthian congregation, who were glorying and puffed up; their glorying was not good, for a little leaven leavens the whole lump. He applies this to the church today, which has accommodated social sins rather than causing a ruckus.
All unrighteousness is sin: and there is a sin not unto death.