Albert Barnes 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 that Jesus is the Son of God. In the true sense, and from the heart. This will always prove that a man is a Christian. But the passage cannot mean that if he merely says so in words, or if he does it insincerely, or without any proper sense of the truth, it will prove that he is a Christian. On the meaning of the sentiment expressed here, see the notes on 1 John 4:2.

Compare the notes on Romans 10:10.