1 John 2: 7-11
7. Brethren, I write no new commandment unto you, but an old commandment which ye had from the beginning. The old commandment is the word which ye have heard from the beginning.
7. Fratres, non mandatum novum mandment scribo vobis, sed mandatum vetus, quod habuistis ab initio: mandatum vetus est sermo quem audistis ab initio.
8. Again, a new commandment I write unto you, which thing is true in him and in you: because the darkness is past, and the true light now shineth.
8. Rursum mandatum novum scribo vobis, quae est veritas in ipso et in vobis; quia tenebrae transeunt, et lumen verum jam lucet.
9. He that saith he is in the light, and hateth his brother, is in darkness even until now.
9. Quia dicit se in luce esse, et fratrem suum odit, in tenebræ est adhuc.
10. He that loveth his brother abideth in the light, and there is none occasion of stumbling in him.
10. Qui diligit fratrem suum, in luce manet, et offendiculum in eo non est.
11. But he that hateth his brother is in darkness, and walketh in darkness, and knoweth not whither he goeth, because that darkness hath blinded his eyes.
11. Qui vero fratrem suum odit, in tenebris ambulat, nec scit quo vadat, quia tenebrae excaecarunt oculos ejus.
7 Brethren, I write no new commandment This is an explanation of the preceding doctrine, that to love God is to keep his commandments. And not without reason did he largely dwell on this point. First, we know that novelty is disliked or suspected. Secondly, we do not easily undertake an unwonted yoke. In addition to these things, when we have embraced any kind of doctrine, we dislike to have anything changed or made new in it. For these reasons John reminds us, that he taught nothing respecting love but what had been heard by the faithful from the beginning, and had by long usage become old.
Some explain oldness differently, even that Christ now prescribes no other rule of life under the Gospel than what God did formerly under the Law. This is indeed most true; nor do I object but that he afterwards calls in this sense the word of the gospel the old commandment But I think that he now means only, that these were the first elements of the gospel, that they had been thus taught from the beginning, that there was no reason why they should refuse that as unusual by which they ought to have been long ago imbued. For the relative seems to be used in a causative sense. He calls it then old, not because it was taught the fathers many ages before, but because it had been taught them on their new entrance into a religious life. And it served much to claim their faith, that it had proceeded from Christ himself from whom they had received the gospel. 
The old commandment The word old, in this place, probably extends further; for the sentence is fuller, when he says, the word which ye have heard from the beginning is the old commandment And as I, indeed, think, he means that the gospel ought not to be received as a doctrine lately born, but what has proceeded from God, and is his eternal truth; as though he had said, "Ye ought not to measure the antiquity of the gospel which is brought to you, by time; since therein is revealed to you the eternal will of God: not only then has God delivered to you this rule of a holy life, when ye were first called to the faith of Christ, but the same has always been prescribed and approved by him." And, doubtless, this only ought to be deemed antiquity, and deserves faith and reverence, which has its origin from God. For the fictions of men, whatever long prescription of years they may have, cannot acquire so much authority as to subvert the truth of God.
8 Again, a new commandment Interpreters do not appear to me to have attained the meaning of the Apostle. He says new, because God, as it were, renews it by daily suggesting it, so that the faithful may practice it through their whole life, for nothing more excellent can be sought for by them. The elements which children learn give place in time to what is higher and more solid. On the contrary, John denies that the doctrine respecting brotherly love is of this kind, is one which grows old with time, but that it is perpetually in force, so that it is no less the highest perfection than the very beginning.
It was, however, necessary that this should be added, for as men are more curious than what they ought to be, there are many who always seek something new. Hence there is a weariness as to simple doctrine, which produces innumerable prodigies of errors, when every one gapes continually for new mysteries. Now, when it is known that the Lord proceeds in the same even course, in order to keep us through life in that which we have learnt, a bridle is cast on desires of this kind. Let him, then, who would reach the goal of wisdom, as to the right way of living, make proficiency in love.
Which then is true, or which is truth. He proves by this reason what he had said; for this one command respecting love, as to our conduct in life, constitutes the whole truth of Christ. Besides, what other greater revelation can be expected? for Christ, doubtless, is the end and the completion of all things. Hence the word truth means this, that they stood, as it were at the goal, for it is to be taken for a completion or a perfect state. He joins Christ to them, as the head to the members, as though he had said, that the body of the Church has no other perfection, or, that they would then be really united to Christ, if holy love existed continually among them.
Some give another explanation, "That which is the truth in Christ, is also in you." But I do not see what the meaning of this is.
Because the darkness is past. The present time is here instead of the past; for he means, that as soon as Christ brings light, we have the full brightness of knowledge: not that every one of the faithful becomes wise the first day as much as he ought to be, (for even Paul testifies that he labored to apprehend what he had not apprehended, (Philippians 3:12,) but that the knowledge of Christ alone is sufficient to dissipate darkness. Hence, daily progress is necessary; and the faith of every one has its dawn before it reaches the noonday. But as God continues the inculcation of the same doctrine, in which he bids us to make advances, the knowledge of the Gospel is justly said to be the true light, when Christ, the Sun of righteousness, shines. Thus the way is shut up against the audacity of those men who try to corrupt the purity of the Gospel by their own fictions; and we may safely denounce an anathema on the whole theology of the Pope, for it wholly obscures the true light.
9 He that saith he is in the light He pursues the same metaphor. He said that love is the only true rule according to which our life is to be formed; he said that this rule or law is presented to us in the Gospel; he said, lastly, that it is there as the meridian light, which ought to be continually looked on. Now, on the other hand, he concludes that all are blind and walk in darkness who are strangers to love. But that he mentioned before the love of God and now the love of the brethren, involves no more contrariety than there is between the effect and its cause. Besides, these are so connected together that they cannot be separated.
John says in the third chapter, that we falsely boast of love to God, except we love our brethren; and this is most true. But he now takes love to the brethren as a testimony by which we prove that we love God. In short, since love so regards God, that in God it embraces men, there is nothing strange in this, that the Apostle, speaking of love, should refer at one time to God, at another to the brethren; and this is what is commonly done in Scripture. The whole perfection of life is often said to consist in the love of God; and again, Paul teaches us, that the whole law is fulfilled by him who loves his neighbor, (Romans 13:8;) and Christ declares that the main points of the law are righteousness, judgment, and truth. (Matthew 23:23.) Both these things are true and agree well together, for the love of God teaches us to love men, and we also in reality prove our love to God by loving men at his command. However this may be, it remains always certain that love is the rule of life. And this ought to be the more carefully noticed, because all choose rather almost anything else than this one commandment of God.
To the same purpose is what follows, and there is no occasion of stumbling in him — that is, in him who acts in love; for, he who thus lives will never stumble. 
11. But he that hateth his brother. He again reminds us, that whatever specious appearance of excellency thou shewest, there is yet nothing but what is sinful if love be absent. This passage may be compared with the thirteenth chapter of the First Epistle to the Corinthians, and no long explanation is needed. But this doctrine is not understood by the world, because the greater part are dazzled by all sorts of masks or disguises. Thus, fictitious sanctity dazzles the eyes of almost all men, while love is neglected, or, at least, driven to the farthest corner.
 That this view is correct, appears evident from the words, "which ye had from the beginning;" he calls it "old," because they had been taught it from "the beginning," that is, of the gospel. Then "new" can mean no other thing than what Calvin states, that it continues still in force, it being, as it were, always new. -- Ed.  Literally, "and to him there is not a stumblingblock;" that is, nothing that causes him to stumble or fall. He is not like him mentioned in the next verse, who "walks in darkness and knows not whither he goeth." The sentence seems to have been taken from Psalm 119:165, with this only difference, that it is "to them," instead of "to him." There is in the Sept no preposition, but in Hebrew the preposition "to" is used; and en has sometimes this meaning in the New Testament. See Colossians 1:23; 1 Thessalonians 4:7. -- Ed.
 Literally, "and to him there is not a stumblingblock;" that is, nothing that causes him to stumble or fall. He is not like him mentioned in the next verse, who "walks in darkness and knows not whither he goeth." The sentence seems to have been taken from Psalm 119:165, with this only difference, that it is "to them," instead of "to him." There is in the Sept no preposition, but in Hebrew the preposition "to" is used; and en has sometimes this meaning in the New Testament. See Colossians 1:23; 1 Thessalonians 4:7. -- Ed.