Love One Another.
By this all will know that you are My disciples, if you have love for one another. John 13:35 NKJV
The reputation of their profession: By this shall all men know that you are my disciples, if you have love one to another. Observe, We must have love, not only show love, but have it in the root and habit of it, and have it when there is not any present occasion to show it have it ready. "Hereby it will appear that you are indeed my followers by following me in this." Note, Brotherly love is the badge of Christ's disciples. By this he knows them, by this they may know themselves (1 John 2:14), and by this others may know them. This is the livery of his family, the distinguishing character of his disciples this he would have them noted for, as that wherein they excelled all others--their loving one another. This was what their Master was famous for all that ever heard of him have heard of his love, his great love and therefore, if you see any people more affectionate one to another than what is common, say, "Certainly these are the followers of Christ, they have been with Jesus." Now by this it appears that the heart of Christ was very much upon it, that his disciples should love one another. In this they must be singular whereas the way of the world is to be every one for himself, they should be hearty for one another. He does not say, By this shall men know that you are my disciples--if you work miracles, for a worker of miracles is but a cypher without charity (1 Corinthians 13:1,2) but if you love one another from a principle of self-denial and gratitude to Christ. This Christ would have to be the proprium of his religion, the principal note of the true church.
That it is the true honour of Christ's disciples to excel in brotherly love. Nothing will be more effectual than this to recommend them to the esteem and respect of others. See what a powerful attractive it was, Acts 2:46,47. Tertullian speaks of it as the glory of the primitive church that the Christians were known by their affection to one another. Their adversaries took notice of it, and said, See how these Christians love one another.
That, if the followers of Christ do not love one another, they not only cast an unjust reproach upon their profession, but give just cause to suspect their own sincerity. O Jesus! are these thy Christians, these passionate, malicious, spiteful, ill-natured people? Is this thy son's coat? When our brethren stand in need of help from us, and we have an opportunity of being service able to them, when they differ in opinion and practice from us, or are any ways rivals with or provoking to us, and so we have an occasion to condescend and forgive, in such cases as this it will be known whether we have this badge of Christ's disciples. - Matthew Henry