Tertullian

“ But if he says, “He who is not righteous is not of God,” [19] how shall he who is not modest again become (a son) of God, who has already ceased to be so?
“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.” ”

“ Every one who doeth righteousness is righteous, as He withal is righteous. He who doeth sin is of the devil, inasmuch as the devil sinneth from the beginning. For unto this end was manifested the Son of God, to undo the works of the devil:” for He has “undone” them withal, by setting man free through baptism, the “handwriting of death” having been “made a gift of” to him: [17] and accordingly, “he who is being born of God doeth not sin, because the seed of God abideth in him; and he cannot sin, because he hath been born of God. Herein are manifest the sons of God and the sons of the devil.” ”

“ 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 ”