New Defender's Study Bible Notes
5:16 sin unto death. The “sin unto death” does not refer to the unforgivable sin, which yields eternal spiritual death. That sin is, ultimately, refusal to accept Jesus Christ as Savior (John 3:36). No other sin precludes at least the possibility of repentance, faith and forgiveness. Therefore, the “sin unto death” can only refer to such flagrant sin on the part of a “brother” that God’s chastisement finally becomes physical death (I Corinthians 11:27-33; 5:5).
5:16 pray for it. There is thus no warrant in praying for the dead.