For the truth’s sake (dia tēn alētheian). Repetition of the word, one of which John is very fond (1Jo 1:6, “the truth, as revealed by the Christ, and gradually unfolded by the Spirit, who is truth” (Brooke).
Which abideth in us (tēn menousan en hēmin). See Joh 17:19 for “sanctified in truth” and 1Jo 2:6 for abiding in Christ, and so it includes all who are in Christ.
It shall be with us (meth' hēmōn estai). Confident assertion, not a mere wish. Note the order of the words, “With us it shall be” (estai future middle of eimi).