1 John 5:19-21
19 And we know that we are of God, and the whole world lieth in wickedness.
19. Novimus quod ex Deo sumus, et mundus torus in maligno positus est.
20. And we know that the Son of God is come, and hath given us an understanding, that we may know him that is true, and we are in him that is true, even in his Son Jesus Christ. This is the true God, and eternal life.
20. Novimus autem quod Filius Dei venit, et dedit nobis intelligentiam, ut cognoscamus illum verum; et sumus in ipso vero, in Filio ejus Jesu Christo: Hic est verus Deus, et vita aeterna.
21. Little children, keep yourselves from idols. Amen.
21. Filioli, custodite vos ab idolis. Amen.
19 We 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.
20 And we know that the Son of God is come As the children of God are assailed on every side, he, as we have said, encourages and exhorts them to persevere in resisting their enemies, and for this reason, because they fight under the banner of God, and certainly know that they are ruled by his Spirit; but he now reminds them where this knowledge is especially to be found.
He then says that God has been so made known to us, that now there is no reason for doubting. The Apostle does not without reason dwell on this point; for except our faith is really founded on God, we shall never stand firm in the contest. For this purpose the Apostle shews that we have obtained through Christ a sure knowledge of the true God, so that we may not fluctuate in uncertainty.
By true God he does not mean one who tells the truth, but him who is really God; and he so calls him to distinguishing him from all idols. Thus true is in opposition to what is fictitious; for it is alethinos, and not alethes A similar passage is in John
"This is eternal life, to know thee,
And he justly ascribes to Christ this office of illuminating our minds as to the knowledge of God. For, as he is the only true image of the invisible God, as he is the only interpreter of the Father, as he is the only guide of life, yea, as he is the life and light of the world and the truth, as soon as we depart from him, we necessarily become vain in our own devices.
And Christ is said to have given us an understanding, not only because he shews us in the gospel what sort of being is the true God, and also illuminates us by his Spirit; but because in Christ himself we have God manifested in the flesh, as Paul says, since in him dwells all the fullness of the Deity, and are hid all the treasures of knowledge and wisdom. (Colossians 2:9.) Thus it is that the face of God in a manner appears to us in Christ; not that there was no knowledge, or a doubtful knowledge of God, before the coming of Christ,, but that now he manifests himself more fully and more clearly. And this is what Paul says in 2 Corinthians 4:6, that
God, who formerly commanded light to shine out of darkness at the creation of the world, hath now shone in our hearts through the brightness of the knowledge of his glory in the face of Christ.
And it must be observed, that this gift is peculiar to the elect. Christ, indeed, kindles for all indiscriminately the torch of his gospel; but all have not the eyes of their minds opened to see it, but on the contrary Satan spreads the veil of blindness over many. Then the Apostle means the light which Christ kindles within in the hearts of his people, and which when once kindled, is never extinguished, though in some it may for a time be smothered.
We are in him that is true By these words he reminds us how efficacious is that knowledge which he mentions, even because by it we are united to Christ; and become one with God; for it has a living root, fixed in the heart, by which it comes that God lives in us and we in him. As he says, without a copulative, that: we are in him that is true, in his Son, he seems to express the manner of our union with God, as though he had said, that we are in God through Christ. 
This is the true God Though the Arians have attempted to elude this passage, and some agree with them at this day, yet we have here a remarkable testimony to the divinity of Christ. The Arians apply this passage to the Father, as though the Apostle should again repeat that he is the true God. But nothing could be more frigid than such a repetition. It has already twice testified that the true God is he who has been made known to us in Christ, why should he again add, This is the true God? It applies, indeed, most suitably to Christ; for after having taught us that Christ is the guide by whose hand we are led to God, he now, by way of amplifying, affirms that Christ is that God, lest we should think that we are to seek further; and he confirms this view by what is added, and eternal life. It is doubtless the same that is spoken of, as being the true God and eternal life. I pass by this, that the relative houtos usually refers to the last person. I say, then, that Christ is properly called eternal life; and that this mode of speaking perpetually occurs in John, no one can deny.
The meaning is, that when we have Christ, we enjoy the true and eternal God, for nowhere else is he to be sought; and, secondly, that we become thus partakers of eternal life, because it is offered to us in Christ though hid in the Father. The origin of life is, indeed, the Father; but the fountain from which we are to draw it, is Christ.
21 Keep yourselves from idols Though this be a separate sentence, yet it is as it were an appendix to the preceding doctrine. For the vivifying light of the Gospel ought to scatter and dissipate, not only darkness, but also all mists, from the minds of the godly. The Apostle not only condemns idolatry, but commands us to beware of all images and idols; by which he intimates, that the worship of God cannot continue uncorrupted and pure whenever men begin to be in love with idols or images. For so innate in us is superstition, that the least occasion will infect us with its contagion. Dry wood will not so easily burn when coals are put under it, as idolatry will lay hold on and engross the minds of men, when an occasion is given to them. And who does not see that images are the sparks? What sparks do I say? nay, rather torches, which are sufficient to set the whole world on fire.
The Apostle at the same time does not only speak of statues, but also of altars, and includes all the instruments of superstitions. Moreover, the Papists are ridiculous, who pervert this passage and apply it to the statues of Jupiter and Mercury and the like, as though the Apostle did not teach generally, that there is a corruption of religion whenever a corporeal form is ascribed to God, or whenever statues and pictures form a part of his worship. Let us then remember that we ought carefully to continue in the spiritual worship of God, so as to banish far from us everything that may turn us aside to gross and carnal superstitions.
end of the first epistle of John
 It is rendered by some, "through his Son Jesus Christ." Our version, "even in his Son Jesus Christ," seems not to be right, as it makes "him that is true," to be the Son, while the reference is to God, as in the previous clause. The true meaning would be thus conveyed, "And we are in the true God, being in his Son Jesus Christ;" for to be in Christ, is to be in God. Three MSS., the Vulgate, and several of the Fathers, read thus, "and we are in his true Son Jesus Christ". -- Ed.