John Calvin 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. He repeats the truth that we are united to God by Christ, and that we cannot be connected with Christ unless God abides in us. Faith and confession are used interchangeably in the same sense; for though hypocrites may cleverly boast of faith, yet the apostle here acknowledges none of those who ordinarily confess, but only those who truly and from the heart believe. Besides, when he says that Jesus is the Son of God, he briefly includes the sum and substance of faith; for there is nothing necessary for salvation that faith does not find in Christ.

After having said, in general, that people are so united to Christ by faith that Christ unites them to God, he added what they themselves had seen, so that he applied a general truth to those to whom he was writing. Then follows the exhortation to love one another as they were loved by God. Therefore, the order and connection of his discourse is this: Faith in Christ makes God dwell in people, and we share in this grace; but as God is love, no one dwells in Him unless he loves his brothers. Then love ought to reign in us, since God unites himself to us.