John Chrysostom — as excerpted in the Catena Aurea on John 13:33-35

“(Hom. lxxii. 4) Passing over the miracles, which they were to perform, He makes love the distinguishing mark of His followers; By this shall all men know that ye are My disciples, if ye have love one to another. This it is that evidences the saint or the disciple, as He calls him.”