A sin unto death seems to be a sin which is committed by a child of God and is so grievous and so offensive to the Lord that God takes his life immediately. An example of this could be taking the Lord's supper without discerning it (1 Corinthians 11.27-30). However, Calvin seems to think that the sin is apostasy from the truth because John says that we know we are of God if we are not continuing in sin.
