What is the Sin unto Death?
John is speaking to those who are Christians. He says, "If any man see his brother sin…." What kind of sin could a Christian commit that would lead to death? James says in James 1:14, 15 "But every man is tempted, when he is drawn away of his own lust, and enticed. Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death." According to James, any temptation can lead to sin and death.