John Calvin's Commentary on the Bible

15 (1 John 4:15)

Verse 15

15Whosoever shall confess He repeats the truth, that we are united to God by Christ, and that we cannot be connected with Christ except, God abides in us. Faith and confession are used indiscriminately in the same sense; for though hypocrites may wisely boast of faith, yet the apostle here acknowledges none of those who ordinarily confess, but such as 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 which faith finds not in Christ

After having said in general, that men are so united to Christ by faith, that Christ unites them to God, he subjoined what they themselves had seen so that he accommodated 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 to dwell in men, and we are partakers of this grace; but as God is love, no one dwells in him except he loves his brethren. Then love ought to reign in us, since God unites himself to us.

- John Calvin's Commentary on the Bible