1 John 5:2

By this we know, &c.— St. John has often intimated, that the love of the Christian brethren, who are the children of God, is a sign or evidence of our love to God; and it appears highly reasonable, that what is visible, should be a sign or evidence of what is invisible. But here he seems to argue the contrary way; namely, that our love to God is a sign or evidence of our love to the children of God, or the Christian brethren. Now it may be objected, "How can what is invisible be looked upon as a sign or evidence of what is visible." In answer towhich, let it be considered, that the friendships of the world are too often confederacies in vice, or leagues in pleasure;andthatChristiansmayloveoneanotherfromnaturalaffection,relationship, temporal interest, or some other worldly motive; but loving them from such considerations, is not that peculiar love of the brethren which the gospel requires. It may be said, "How then shall we know that we love them spiritually and as Christians?" The apostle has here answered that very question: for, having declared, 1 John 5:1 that he who loveth God, the Father of Christians, is obliged to love Christians, who are his children, he here adds, "By this we may know that our love to Christians is of the right sort, when it proceeds from a love to God, and a sincere desire to keep his commandments; among which this of loving the Christian brethren is none of the least." A love to Christians, which has an extensive piety and virtue for its basis, must be highly valuable; Matthew 12:50. A man who lives in any vice, or who does not so love God, as to make conscience of keeping all his commandments, may be assured that his love to Christians is not of the right sort: but wherever there is extensive virtue and piety, there is the best proof of the genuineness of any one single grace or virtue, and particularly of our love to Christians, who are the children of God.