A.T. Robertson Commentary 2 John 1:2

"for the truth`s sake which abideth in us, and it shall be with us for ever:" — 2 John 1:2 (ASV)

For the truth's sake (δια την αληθειαν). Repetition of the word, one of which John is very fond (1 John 1:6, "the truth, as revealed by the Christ, and gradually unfolded by the Spirit, who is truth" (Brooke).

Which abideth in us (την μενουσαν εν ημιν). See Joh 17:19 for "sanctified in truth" and 1 John 2:6 for abiding in Christ, and so it includes all who are in Christ.

It shall be with us (μεθ' ημων εστα). Confident assertion, not a mere wish. Note the order of the words, "With us it shall be" (εστα future middle of ειμ).