A.T. Robertson Commentary 1 John 2:5

"but whoso keepeth his word, in him verily hath the love of God been perfected. Hereby we know that we are in him:" — 1 John 2:5 (ASV)

But whoso keepeth (ος δ' αν τηρη). Indefinite relative clause with modal αν and the present active subjunctive, "whoever keeps on keeping."

Verily (αληθως). Truly, of a truth. This prize is open to all, not confined to a few initiated Gnostic intellectuals or pneumatics.

Hath the love of God been perfected (η αγαπη του θεου τετελειωτα). Perfect passive indicative of τελειοω, stands completed. Probably objective genitive, our love for God, which is realized in absolute obedience (Brooke).

Hereby (εν τουτω). That is by continuous keeping of Christ's commandments, not by loud talk and loose living.