1 John 2:7

The seventh verse may be supposed either to look backward to what immediately preceded (and then it is walking as Christ walked that is here represented as no new, but an old commandment; it is that which the apostles would certainly inculcate wherever they brought Christ's gospel), or to look forward to what the apostle is now going to recommend, and that is the law of fraternal love; this is the message heard from the beginning (Jo1 3:11), and the old commandment, Jo2 1:5. Now, while the apostle addresses himself to the recommendation of such a practice, he is ready to give an instance thereof in his affectionate appellation: "Brethren, you who are dear to me in the bond of that love to which I would solicit you;" and so the precept of fraternal love is recommended, I. As an old one: I write no new commandment unto you, but an old commandment, which you had from the beginning, Jo1 2:7. The precept of love must be as old as human nature; but it might admit divers enactions, enforcements, and motives. In the state of innocence, had human nature then been propagated, men must have loved one another as being of one blood, made to dwell on the earth, as being God's offspring, and bearing his image. In the state of sin and promised recovery, they must love one another as related to God their Maker, as related to each other by blood, and as partners in the same hope.

— Matthew Henry's Concise Commentary (public domain)

Brethren--The oldest manuscripts and versions read instead, "Beloved," appropriate to the subject here, love. no new commandment--namely, love, the main principle of walking as Christ walked (Jo1 2:6), and that commandment, of which one exemplification is presently given, Jo1 2:9-10, the love of brethren. ye had from the beginning--from the time that ye first heard the Gospel word preached.

— Jamieson-Fausset-Brown Commentary (public domain)