“If we say we have no sin, we deceive ourselves, and the truth is not in us.”—1 John 1:8.
“Whosoever is born of God doth not commit sin, for his seed remaineth in him; and he cannot sin, because he is born of God.”—1 John 3:9.
It appears that the word sin, in these passages, is of different significations. In the former it is to be taken properly, for any transgression of the law of God. If any man say, in this sense, he has no sin, he only proves himself to be deceived, and that he has yet to learn what is true religion.
But in the latter, it seems, from the context, that the term is intended to denote the sin of apostacy. If we were to substitute the term apostacy for sin, from the sixth to the tenth verse, the meaning would be clear. Whoso abideth in him apostatizeth not: whosoever apostatizeth hath not seen him, neither known him.—He that is guilty of apostacy is of the devil; for the devil hath been an apostate from the beginning.—Whosoever is born of God doth not apostatize; for his seed remaineth in him: and he cannot apostatize, because he is born of God.
This sense of the latter passage perfectly agrees with what is said of the “sin unto death,” ver. 16–18. “There is a sin unto death.… We know that whosoever is born of God sinneth not; but he that is begotten of God keepeth himself, and that wicked one toucheth him not.” It also agrees with chap. 2:19, “They went out from us, but they were not of us; for if they had been of us, they would, no doubt, have continued with us. But they went out, that they might be made manifest that they were not all of us.” Altogether, it affords what we might presume to call an incontestable proof of the certain perseverance of true believers.
Fuller, A. G. (1988). The Complete Works of Andrew Fuller: Memoirs, Sermons, Etc. (J. Belcher, Ed.) (Vol. 1, pp. 667–684). Harrisonburg, VA: Sprinkle Publications.