As for you (humeis). Emphatic proleptic position before the relative ho and subject of ēkousate, a familiar idiom in Joh 8:45; Joh 10:29, etc. Here for emphatic contrast with the antichrists. See 1Jo 1:1 for ap' archēs (from the beginning).
Let abide in you (en humin menetō). Present active imperative of menō, to remain. Do not be carried away by the new-fangled Gnostic teaching.