What is a sin that is “unto death”?

John writes, “If any man see his brother sin a sin which is not unto death, he shall ask, and he shall give him life for them that sin not unto death. There is a sin unto death: I do not say that he shall pray for it. All unrighteousness is sin: and there is a sin not unto death” (1 John 5:16–17, kjv).

John is saying that when we see fellow believers sinning, some of them are sinning unto death (meaning that they will go on sinning and will not repent), while others are not sinning unto death (meaning that they are not sinning maliciously and will repent and have their record washed clean). John is also saying that our prayers will benefit those whose sins are not unto death but will accomplish nothing for those whose sins are unto death. I don’t believe John is telling his readers not to pray for those who have abandoned the faith. Rather, I think he is telling them they should realize that salvation is conditional; it is granted only to those who repent of their sins. Thus, if our prayers are to be beneficial, the person we’re praying for has to be willing to repent and reform his life. John wants his readers to be aware of that fact.

God forgives sins upon repentance, and He helps straying believers come to repentance, but He does not take over their wills; He does not reach into their minds, take hold of the “willer,” and cause them to repent. The sinner himself must be a willing participant. The choice is left to him.