1 John 5:19

19We are of God He deduces an exhortation from his previous doctrine; for what he had declared in common as to the children of God, he now applies to those he was writing to; and this he did, to stimulate them to beware of sin, and to encourage them to repel the onsets of Satan.

Let readers observe, that it is only true faith, that applies to us, so to speak, the grace of God; for the Apostle acknowledges none as faithful, but those who have the dignity of being God’s children. Nor does he indeed put probable conjecture, as the Sophists speak, for confidence; for he says that we know. The meaning is, that as we have been born of God, we ought to strive to prove by our separation from the world, and by the sanctity of our life, that we have not been in vain called to so great all honor.

Now, this is an admonition very necessary for all the godly; for wherever they turn their eyes, Satan has his allurements prepared, by which he seeks to draw them away from God. It would then be difficult for them to hold on in their course, were they not so to value their calling as to disregard all the hindrances of the world. Then, in order to be well prepared for the contest, these two things must be borne in mind, that the world is wicked, and that our calling is from God.

Under the term world, the Apostle no doubt includes the whole human race. By saying that it lieth in the wicked one, he represents it as being under the dominion of Satan. There is then no reason why we should hesitate to shun the world, which condemns God and delivers up itself into the bondage of Satan: nor is there a reason why we should fear its enmity, because it is alienated from God. In short, since corruption pervades all nature, the faithful ought to study self-denial; and since nothing is seen in the world but wickedness and corruption, they must necessarily disregard flesh and blood that they may follow God. At the same time the other thing ought to be added, that God is he who has called them, that under this protection they may oppose all the machinations of the world and Satan.