The basic problem in v.2 is that it has a participle μένουσαν in apparent coordination with a finite verb ἔσται. Most translations seem to treat this apparent coordination as real, almost as if ἔσται can be replaced by a future participle (perhaps ἐσομένην as in Classical Attic?), or, as BDF § 442(6) suggests, as if the participle τὴν μένουσαν read ἣ . . . μένει. Thus, they translate accordingly: "because of the truth that abides in us and will be with us forever" (NRSV).2 John 1-2 wrote:1 Ὁ πρεσβύτερος ἐκλεκτῇ κυρίᾳ καὶ τοῖς τέκνοις αὐτῆς, οὓς ἐγὼ ἀγαπῶ ἐν ἀληθείᾳ, καὶ οὐκ ἐγὼ μόνος ἀλλὰ καὶ πάντες οἱ ἐγνωκότες τὴν ἀλήθειαν, 2 διὰ τὴν ἀλήθειαν τὴν μένουσαν ἐν ἡμῖν καὶ μεθ’ ἡμῶν ἔσται εἰς τὸν αἰῶνα.
BDF further claims that the use of καί to coordinate words with independent clauses is "Hebraizing." An alternative explanation in BDF § 468(3) is that it is an example of an anacoluthon between a participle and a finite verb. These explanations seem to be beg the question that the NA27/USB4 punctuation is correct, but however close in thought v.2b is to the participial clause, it is syntactically manifested as an independent clause and the I feel the punctuation should reflect that. I don't think it is necessary to appeal to a supposed Hebraizing or anacoluthon to account for the syntax.
Thus, it seems more straightforward to me if the v.2b was simply treated as a separate independent clause: "And it will forever be with us." Remarkably, this option is rarely adopted in the translations I consult, only the ASV and the ERV of the late 19th century (plus the New International Reader's Version), even among the usual "dynamic equivalent" suspects who are used to chopping up the Greek sentence.
Thoughts?