The apostle, having mentioned keeping the commandments, and pleasing God, as the qualification of effectual petitioners in and with Heaven, here suitably proceeds,
I. To represent to us what those commandments primarily and summarily are; they are comprehended in this double one: And this is his commandment, That we should believe on the name of his Son Jesus Christ, and love one another, as he gave us commandment, Jo1 3:23. To believe on the name of his Son Jesus Christ is, 1. To discern what he is, according to his name, to have an intellectual view of his person and office, as the Son of God, and the anointed Saviour of the world. That every one that seeth the Son, and believeth on him, may have everlasting life, Joh 6:40. 2. To approve him in judgment and conscience, in conviction and consciousness of our case, as one wisely and wonderfully prepared and adapted for the whole work of eternal salvation. 3. To consent to him, and acquiesce in him, as our Redeemer and recoverer unto God. 4. To trust to him, and rely upon him, for the full and final discharge of his saving office. Those that know thy name will put their trust in thee, Psa 9:10. I know whom I have believed, and I am persuaded that he is able to keep that which I have committed unto him against that day, Ti2 1:12. This faith is a needful requisite to those who would be prevalent petitioners with God, because it is by the Son that we must come to the Father; through his grace and righteousness our persons must be accepted or ingratiated with the Father (Eph 1:6), through his purchase all our desired blessings must come, and through his intercession our prayers must be heard and answered. This is the first part of the commandment that must be observed by acceptable worshippers; the second is that we love one another, as he gave us commandment, Jo1 3:23. The command of Christ should be continually before our eyes. Christian love must possess our soul when we go to God in prayer. To this end we must remember that our Lord obliges us, (1.) To forgive those who offend us (Mat 6:14), and, (2.) To reconcile ourselves to those whom we have offended, Mat 5:23, Mat 5:24. As good-will to men was proclaimed from heaven, so good-will to men, and particularly to the brethren, must be carried in the hearts of those who go to God and heaven.
II. To represent to us the blessedness of obedience to these commands. The obedient enjoy communion with God: And he that keepeth his commandments, and particularly those of faith and love, dwelleth in him, and he in him, Jo1 3:24. We dwell in God by a happy relation to him, and spiritual union with him, through his Son, and by a holy converse with him; and God dwells in us by his word, and our faith fixed on him, and by the operations of his Spirit. Then there occurs the trial of his divine inhabitation: And hereby we know that he abideth in us, by the Spirit which he hath given us (Jo1 3:24), by the sacred disposition and frame of soul that he hath conferred upon us, which being a spirit of faith in God and Christ, and of love to God and man, appears to be of God.

Dwelleth in him - i.e. in God; and he - God, in him - the believer.
And hereby we know - We know by the Spirit which he hath given us that we dwell in God, and God in us. It was not by conjecture or inference that Christians of old knew they were in the favor of God, it was by the testimony of God's own Spirit in their hearts; and this testimony was not given in a transient manner, but was constant and abiding while they continued under the influence of that faith that worketh by love. Every good man is a temple of the Holy Ghost, and wherever he is, he is both light and power. By his power he works; by his light he makes both himself and his work known. Peace of conscience and joy in the Holy Ghost must proceed from the indwelling of that Holy Spirit; and those who have these blessings must know that they have them, for we cannot have heavenly peace and heavenly joy without knowing that we have them. But this Spirit in the soul of a believer is not only manifest by its effects, but it bears its own witness to its own indwelling. So that a man not only knows that he has this Spirit from the fruits of the Spirit, but he knows that he has it from its own direct witness. It may be said, "How can these things be?" And it may be answered, By the power, light, and mercy of God. But that such things are, the Scriptures uniformly attest, and the experience of the whole genuine Church of Christ, and of every truly converted soul, sufficiently proves. As the wind bloweth where it listeth, and we cannot tell whence it cometh and whither it goeth, so is every one that is born of the Spirit: the thing is certain, and fully known by its effects; but how this testimony is given and confirmed is inexplicable. Every good man feels it, and knows he is of God by the Spirit which God has given him.

And he that keepeth his commandments,.... Attends to those instructions and declarations concerning faith in Christ, and love to the brethren, and acts according to them:

dwelleth in him, and he in them; that is, he dwells in Christ, and Christ dwells in him; the same is said of believing in Christ under the figurative expressions of eating his flesh, and drinking his blood; see Gill on Joh 6:56;

and hereby we know that he abideth in us; or dwelleth in us, as before,

by the Spirit which he hath given us; which if understood of private Christians, as the preceding verses incline to, the sense is, that union to Christ, and the continuance of it, or his indwelling as a fruit of union, and the permanency of that, are evidenced by the Spirit of God; who is given in consequence of union and relation to Christ, as a spirit of regeneration and sanctification, of faith and love, of adoption, and as the earnest of the heavenly inheritance; but if of the apostles and ministers of the word, it may regard the gifts of the Holy Spirit bestowed on them, fitting them for their work and office, and who is a spirit of truth, and not of error; and by having and enjoying these, they knew that Christ abode in them, and had reason to believe, according to his promise, that he would be with them, and with his ministering: servants in succession, to the end of the world; and this sense seems to be encouraged by the former part of the following chapter.

Next: 1 John Chapter 4

dwelleth in him--The believer dwelleth in Christ.

and he in him--Christ in the believer. Reciprocity. "Thus he returns to the great keynote of the Epistle, abide in Him, with which the former part concluded" (Jo1 2:28).

hereby--herein we (believers) know that he abideth in us, namely, from (the presence in us of) the Spirit "which He hath given us." Thus he prepares, by the mention of the true Spirit, for the transition to the false "spirit," Jo1 4:1-6; after which he returns again to the subject of love.

Next: 1 John Chapter 4