1 John 3:24

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 John 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.