Whosoever shall confess that Jesus is the Son of God, God abideth in him, and he in God.
Faith and confession are one and the same thing here, not mere words but belief from the heart. When a man truly confesses Jesus as the Son of God, he has grasped the sum of all salvation, and in that instant God dwells in him and he dwells in God. Because God is love, whoever is joined to Him must love his brethren.
AI summary
A bare assent means nothing; devils and unregenerate men have confessed that Jesus is the Son of God. True confession is belief from the heart unto righteousness, plus cheerful obedience to His commands from love and faith. Only such confession brings the indwelling of God.
AI summary
shall confess--once for all: so the Greek aorist means. that Jesus is the Son of God--and therefore "the Saviour of the world" (Jo1 4:14).