Problem: On the one hand, the Scriptures speak of God’s free and unconditional forgiveness to all who want it (cf. Acts 13:38–39; Rom. 5:20; 1 John 2:1). On the other hand, Jesus spoke of an unpardonable sin that can never be forgiven. And John declares here that there is a “sin leading to death.”
Solution: Bible commentators differ on just what John had in mind here. Some say he was referring to repeated sin (see comments on 1 John 3:9). Others believe he was speaking of a grave sin. Still others believe he had apostasy in mind (cf. 2 Peter 2).
Whatever John envisioned, there is no reason that it could not refer to a sin so serious that it would eventuate in physical death. Paul mentioned that the Corinthians had so participated in the Lord’s Supper in an unworthy manner that some were sick and others were dead as a result (1 Cor. 11:29–30). In fact, the priests Nadab and Abihu were smitten dead for their disobedience to the Lord (Num. 3:4), as were Ananias and Saphira for their sin (Acts 5:1–9). So, it is entirely possible that John has some such serious sin in mind here whereby the believer is turned over to Satan for “the destruction of the flesh, that his spirit may be saved in the day of the Lord Jesus” (1 Cor. 5:5).