Objections from the Revelation and the First Epistle of St. John Refuted
“It is therefore nearly equivalent to saying that John has forgotten himself; asserting, in the former part of his Epistle, that we are not without sin, but now prescribing that we do not sin at all: and in the one case flattering us somewhat with hope of pardon, but in the other asserting with all stringency, that whoever may have sinned are no sons of God.” But away with (the thought): for not even we ourselves forget the distinction between sins, which was the starting-point of our digression. And (a right distinction it was); for John has here sanctioned it; in that there are some sins of daily committal, to which we all are liable: for who will be free from the accident of either being angry unjustly, and retaining his anger beyond sunset; or else even using manual violence or else carelessly speaking evil; or else rashly swearing; or else forfeiting his plighted word or else lying, from bashfulness or “necessity?” In businesses, in official duties, in trade, in food, in sight, in hearing, by how great temptations are we plied! So that, if there were no pardon for such sins as these, salvation would be unattainable to any. Of these, then, there will be pardon, through the successful Suppliant of the Father, Christ. But there are, too, the contraries of these; as the graver and destructive ones, such as are incapable of pardon— murder, idolatry, fraud, apostasy, blasphemy; (and), of course, too, adultery and fornication; and if there be any other “violation of the temple of God.” For these Christ will no more be the successful Pleader: these will not at all be incurred by one who has been born of God, who will cease to be the son of God if he do incur them.
Thus John's rule of diversity will be established; arranging as he does a distinction of sins, while he now admits and now denies that the sons of God sin. For (in making these assertions) he was looking forward to the final clause of his letter, and for that (final clause) he was laying his preliminary bases; intending to say, in the end, more manifestly: “If any knows his brother to be sinning a sin not unto death, he shall make request, and the Lord shall give life to him who sins not unto death. For there is a sin unto death: not concerning that do I say that one should make request.” He, too, (as I have been), was mindful that Jeremiah had been prohibited by God to deprecate (Him) on behalf of a people which was committing mortal sins. “Every unrighteousness is sin; and there is a sin unto death. But we know that every one who has been born of God sins not” — to wit, the sin which is unto death. Thus there is no course left for you, but either to deny that adultery and fornication are mortal sins; or else to confess them irremissible, for which it is not permitted even to make successful intercession.
Source: On Modesty (New Advent)