And he that keepeth his commandments dwelleth in him, and he in him. And hereby we know that he abideth in us, by the Spirit which he hath given us. 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.
Commentary on the Bible, by Adam Clarke [1831].