A.T. Robertson Commentary 1 John 4:15

"Whosoever shall confess that Jesus is the Son of God, God abideth in him, and he in God." — 1 John 4:15 (ASV)

Whosoever shall confess (ος εαν ομολογηση). Indefinite relative clause with modal εαν (=an) and the first aorist active subjunctive, "whoever confesses." See 2:23; 4:2f. for ομολογεω.

That (οτ). Object clause (indirect assertion) after ομολογεω. This confession of the deity of Jesus Christ implies surrender and obedience also, not mere lip service (cf. 1 Corinthians 12:3; Romans 10:6–12). This confession is proof (if genuine) of the fellowship with God (1:3f.; 3:24).