John Calvin Commentary 1 John 4:21

"And this commandment have we from him, that he who loveth God love his brother also." — 1 John 4:21 (ASV)

And this commandment This is a stronger argument, drawn from the authority and doctrine of Christ; for he not only gave a commandment concerning the love of God, but also commanded us to love our brothers. Therefore, we must begin with God in such a way that, at the same time, a transition is made to others.