But whoso keepeth (hos d' an tērēi). Indefinite relative clause with modal an and the present active subjunctive, “whoever keeps on keeping.”
Verily (alēthōs). 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 (hē agapē tou theou teteleiōtai). Perfect passive indicative of teleioō, stands completed. Probably objective genitive, our love for God, which is realized in absolute obedience (Brooke).
Hereby (en toutōi). That is by continuous keeping of Christ’s commandments, not by loud talk and loose living.