John Calvin Reformed @genevareformer

21 And this commandment This is a stronger argument, drawn from the authority and doctrine of Christ; for he not only gave a commandment respecting the love of God, but bade us also to love our brethren. We must therefore so begin with God, as that there may be at the same time a transition made to men. CHAPTER 5 1 John 5:1-5 1.

Matthew Henry Presbyterian @wholebiblehenry

The apostle, having thus excited and enforced sacred love from the great pattern and motive of it, the love that is and dwells in God himself, proceeds to recommend it further by other considerations; and he recommends it in both the branches of it, both as love to God, and love to our brother or Christian neighbour. I.

Commenting on 1 John 4:17-21

Albert Barnes Presbyterian @notesbybarnes

Verse 21. And this commandment have we from him. That is, the command to love a brother is as obligatory as that to love God. If one is obeyed, the other ought to be also; if a man feels that one is binding on him, he should feel that the other is also; and he can never have evidence that he is a true Christian...