5:17. All iniquity is sin. And there is a sin unto death.
5:18. We know that whosoever is born of God sinneth not: but the generation of God preserveth him and the wicked one toucheth him not.
5:19. We know that we are of God and the whole world is seated in wickedness.
And the whole world is seated in wickedness… That is, a great part of the world. It may also signify, is under the wicked one, meaning the devil, who is elsewhere called the prince of this world, that is, of all the wicked. John 12.31.
5:20. And we know that the Son of God is come. And he hath given us understanding that we may know the true God and may be in his true Son. This is the true God and life eternal.
And may be in his true Son. He is, or this is the true God, and life eternal… Which words are a clear proof of Christ's divinity, and as such made use of by the ancient fathers.
5:21. Little children, keep yourselves from idols. Amen.
Keep yourselves from idols… An admonition to the newly converted Christians, lest conversing with heathens and idolaters, they might fall back into the sin of idolatry, which may be the sin unto death here mentioned by St. John.