William Burkitt on 1 John 4:21

This commandment; this great and chief commandment, above the rest, this summary and comprehensive commandment, including all the rest, namely, to love God above all, for his own sake, and to love our brother as ourselves, for God's sake, this command, so full of wisdom, so agreeable to right reason, and so much our duty and interest to comply with, have we received from God; and it is most certain that we love him not, if we keep it not; This commandment have we from God, that he who loveth God, loveth his brother also.

Learn thence, 1. That the great God, by his gracious command, requires that we love him and place the supremacy of our love upon him.

2. That as God requires us to love him above ourselves, so does he oblige us, by virtue of his command, to love our brother as ourselves; as sincerely, though not so intensely, as ourselves.

Learn, 3. That the same commandment that requires us to love God, requires love to our brethren also; God interprets the neglect of our duty to our brother, as a neglect of our obligation to himself.