Jamieson, Fausset & Brown

Not merely repeating the proposition, Jo1 2:3, or asserting the merely opposite alternative to Jo1 2:4, but expanding the "know Him" of Jo1 2:3, into "in Him, verily (not as a matter of vain boasting) is the love of (that is towards) God perfected," and "we are in Him." Love here answers to knowledge in Jo1 2:3. In proportion as we love God, in that same proportion we know Him, and vice versa, until our love and knowledge shall attain their full maturity of perfection. his word--His word is one (see on Jo1 1:5), and comprises His "commandments," which are many (Jo1 2:3). hereby--in our progressing towards this ideal of perfected love and obedience. There is a gradation: Jo1 2:3, "know Him"; Jo1 2:5, "we are in Him"; Jo1 2:6, "abideth in Him"; respectively, knowledge, fellowship, abiding constancy. [BENGEL].