3. this hope—of being hereafter "like Him." Faith and love, as well as hope, occur in 1Jo 3:11, 23.
in—rather, "(resting) upon Him"; grounded on His promises.
purifieth himself—by Christ's Spirit in him (Joh 15:5, end). "Thou purifiest thyself, not of thyself, but of Him who comes that He may dwell in thee" [Augustine]. One's justification through faith is presupposed.
as he is pure—unsullied with any uncleanness. The Second Person, by whom both the Law and Gospel were given.