For the truth's sake - They love this family because they love the truth, and see it so cordially embraced and so happily exemplified. Those who love the gospel itself will rejoice in all the effects which it produces in society, on individuals, families, neighborhoods, and their hearts will be drawn with warm affection to the places where its influence is most fully seen.
Which dwelleth in us - In us who are Christians; that is, the truths of the gospel which we have embraced. Truth may be said to have taken up a permanent abode in the hearts of all who love religion.
And shall be with us for ever - Its abode with us is not for a night or a day; not for a month or a year; not for the few years that make up mortal life; it is not a passing stranger that finds a lodging like the weary traveler for a night, and in the morning is gone to be seen no more; it has come to us to make our hearts its permanent home, and it is to be with us in all worlds, and while ceaseless ages shall roll away.

For the truth's sake - On account of the Gospel.
Which dwelleth in us - By the grace which it has proclaimed.
And shall be with us - For God will preserve not only the Christian religion but its truth, all its essential doctrines for ever. And they that abide in the truth shall go whither that truth leads, i.e. to glory. The Armenian has a strange reading here: "For the truth's sake which dwelleth in us, because it is also with you; and ye shall be with us for ever." But this is supported by no other version, nor by any MS.

For the truth's sake, which dwelleth in us,.... Not for her high birth, nobility, or riches; but either for Christ's sake, who is the truth, and who dwells in the hearts of believers by faith, and who is the same that dwells in one as in another; and on his account it is that saints love one another, because they belong to him, he is formed in them, and his image is stamped upon them; and every like loves its like: or for the Gospel's sake, which has a place, and dwells in every saint, and is the same for matter and substance in one as in another; and unity of mind and judgment produces unity of affection: or for the sake of the truth of grace, the inward principle of grace, which dwells in every regenerate person; a communication of the experience of which knits the saints one to another:
and shall be with us for ever; where Christ enters and takes up his abode, from thence he never finally and totally departs, though he may sometimes hide his face with respect to communion, or withdraw his gracious presence; and where the Gospel has once took place in the heart, and is become the ingrafted word, it can never be rooted out, or be removed; and where the truth of grace is, it will remain; it is an incorruptible seed, a well of living water, springing up into eternal life.

For the truth's sake--joined with "I love," 2-John 1:1. "They who love in the truth, also love on account of the truth."
dwelleth in us, and shall be with us for ever--in consonance with Christ's promise.

For the truth's sake, which abideth in us - As a living principle of faith and holiness.