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.

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, unless he manifests love to his brethren as well as love to God. See the notes at James 2:10.
That he who loveth God love his brother also - See the notes at John 13:34-35. Compare John 15:12, John 15:17.

This commandment have we - We should love one another, and love our neighbor as ourselves. The love of God and the love of man can never be separated; he who loves God will love his brother; he who loves his brother gives this proof that he loves God, because he loves with a measure of that love which, in its infinitude, dwells in God.

(17) And this commandment have we from him, That he who loveth God love his brother also.
(17) A second reason, why God cannot be hated and our neighbour loved, because this same lawmaker commanded us both to love him and our neighbour.

And this commandment have we from him,.... Either "from God", as the Alexandrian copy and the Vulgate Latin version read; and that to love the brethren is a commandment of God, is clear from 1-John 3:23; or from Christ, for it is also a command of his, even his new commandment, which he has given, and his people have received from him:
that he who loveth God, love his brother also; see John 13:34; which is an argument persuading to attend to the one as well as to the other; for the same command that requires the one, requires the other: and he that transgresses it in one case, is a transgressor of it, as well as in the other.

Besides the argument (1-John 4:20) from the common feeling of men, he here adds a stronger one from God's express commandment (Matthew 22:39). He who loves, will do what the object of his love wishes.
he who loveth God--he who wishes to be regarded by God as loving Him.

And this commandment have we from him - Both God and Christ. That he who loveth God love his brother - Every one, whatever his opinions or mode of worship be, purely because he is the child, and bears the image, of God. Bigotry is properly the want of this pure and universal love. A bigot only loves those who embrace his opinions, and receive his way of worship; and he loves them for that, and not for Christ's sake.