Does I John 5:16 teach "Mortal Sin"?

16-17. A most appropriate prayer for the Christian is for the forgiveness of his brother’s sins, that God “may give him life” when he has put himself in danger of eternal death; this prayer serves as a good example of what is “according to his will.” The author does not counsel prayer, however, for one who has sinned “unto death,” for the presumption is that it is not according to God’s will to pardon such a person. all unrighteousness is sin, but there is a sin not unto death: By a sin “unto death” the author evidently means some extraordinary sin (not necessarily specified in his own mind or in those of his readers) so terrible to contemplate that forgiveness, morally speaking, cannot be expected (cf. Str-B 3, 779). He does not mean simply mortal sin as distinguished from venial sin, for the “sins not unto death” in this context also include mortal sins. It is possible that he has in mind the activity of the “Antichrists” previously mentioned (2:18-29). The NT singles out various sins as being in such a category apart (cf. Mk 3:29 par.; Heb 6:4-8; 10:26-31).​

Brown, R. E., Fitzmyer, J. A., & Murphy, R. E. (1996, c1968). The Jerome Biblical commentary. Englewood Cliffs, N.J.: Prentice-Hall.

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