If anyone sees his brother sinning a sin which does not lead to death, he will ask, and He will give him life for those who commit sin not leading to death. There is sin leading to death. I do not say that he should pray about that. All unrighteousness is sin, and there is sin not leading to death. We know that whoever is born of God does not sin, but he who has been born of God keeps himself, and the wicked one does not touch him. We know that we are of God, and the whole world lies under the sway of the wicked one. And we know that the Son of God has come and has given us an understanding, that we may know Him who is true; and we are in Him who is true, in His Son Jesus Christ. This is the true God and eternal life. Little children, keep yourselves from idols. Amen. (1Jo 5:16-21 NKJ)

John here, having implied that sins committed by believers (sins “not resulting in death”) may be prayed for and forgiven. He doesn’t want to give the impression that such sin is insignificant, all sin is unrighteousness, and God doesn’t make concessions.  Such a sin, even though it has been forgiven, may have a natural consequence.

So, what are those sins unto death?  These would be the sins of heresy or attributing the work of the Holy Spirit to the devil.  These sins lead to death because their very action cut off the person and make them an unbeliever.