D-R Bible, Haydock Commentary:
Ver. 16. A sin which is not unto death…and life shall be given to him. It is hard to determine what St. John here calls a sin which is not unto death, and a sin which is unto death. The difference cannot be the same as betwixt sins that are called venial and mortal; for he says, that if a man pray for his brother who commits a sin that is not unto death, life shall be given to him: therefore such a one had before lost the life of grace, and been guilty of what is commonly called a mortal sin. And when he speaks of a sin that is unto death, and adds these words, I do not say that any one should ask for that sin, it cannot be supposed that St. John would say this of every mortal sin, but only of some heinous sins which are very seldom remitted, because such sinners very seldom repent. By a sin therefore which is not unto death, interpreters commonly understand a wilful apostacy from the faith, and from the known truth, when a sinner hardened by his own ingratitude becomes deaf to all admonitions, will do nothing for himself, but runs on to final impenitence. Nor yet does St. John say that such a sin is never remitted, or cannot be remitted, but only has these words, I do not say that any one should ask for the remission of that sin; that is, though we must pray for all sinners whatsoever, yet man cannot pray for such sinners with such a confidence of obtaining always their petitions, as St. John said before, ver. 14. Whatever exposition we follow on this verse, our faith teaches us from the holy Scriptures, that God desires not the death of any sinner, but that he be converted and live. See Ezechiel xxxiii. 11. Though men’s “sins be as the scarlet, they shall be made as white as snow.” (Isaias i. 18.) It is the will of God that every one come to the knowledge of truth and be saved. See John vi. 40. There is no sin so great but which God is willing to forgive, and has left power in his Church to remit the most enormous sins; so that no sinner need despair of pardon, nor will any sinner perish but by his own fault. (Witham) — A sin unto death. Some understand this of final impenitence, or of dying in mortal sin, which is the only sin that never can be remitted; but, it is probable, he may also comprise under this name the sin of apostacy from the faith, and some other such henious sins as are seldom and hardly remitted: and therefore he gives little encouragement to such as pray for these sinners, to expect what they ask. (Challoner)
Ver. 17. All iniquity[4] is sin. The sense here is, that sin is always an injury or an injustice done to God; but though every sin implies such an injury and an offence against God, yet there are different degrees in such injuries, which are not always such an injustice as St. John calls the sin unto death. (Witham)
