As for you (υμεις). Emphatic proleptic position before the relative ο and subject of ηκουσατε, a familiar idiom in John 8:45; John 10:29, etc. Here for emphatic contrast with the antichrists. See 1. John 1:1 for απ' αρχης (from the beginning).

Let abide in you (εν υμιν μενετω). Present active imperative of μενω, to remain. Do not be carried away by the new-fangled Gnostic teaching.