Expositor's Bible Commentary Commentary 1 John 5:1

"Whosoever believeth that Jesus is the Christ is begotten of God: and whosoever loveth him that begat loveth him also that is begotten of him." — 1 John 5:1 (ASV)

The argument parallels 4:19. Even as we love only because God first loved us, so also our belief is possible only because we have first been “born of God.” The author is not addressing the question of incorporation into the family of God but is rather looking only at its result. “Believing” in Jesus is a direct consequence of our “having been born” of God and therefore becomes a test or proof of that birth. From this the author moves to a truism from nature: whoever loves one’s progenitor will also love those similarly born, even one’s brothers and sisters.