▼5:16 A sin which is not to death, etc: It is hard to determine what St. John here calls a sin which is not to death, and a sin which is unto death. The difference can not 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 to death, life shall be given 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, for that I say not that any man ask, 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 unto death, interpreters commonly understand a wilfull apostasy 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 a final impenitence. Nor yet does St. John say, that such a sin is never remitted, or cannot be remitted, but only has these words, for that I say not that any man ask the remission : that is, though we must pray for all sinners whatsoever, yet men can not 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, Ezech. 33.11. Though men’s sins be as red as scarlet, they shall become as white as snow, Isa. 3.18. It is the will of God that every one come to the knowledge of the truth, and be saved. There is no sin so great but which God is willing to forgive, and has left a 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.(Challoner)

▼5:16 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 apostasy from the faith, and some other such heinous 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)