John 13:35 - Ellicott's Commentary On The Whole Bible

By this shall all men know that ye are my disciples. — The thought of their state of orphanage when He should depart from them is still present. He gives them a bond of union, by which they should always be linked to Him and to each other in the principle of love. The followers of great Teachers and Rabbis had their distinctive marks. Here was the distinctive Christian mark, which all men should be able to read. It is instructive that the characteristic mark of Christianity should thus be asserted by its Founder to consist, not in any formulary or signs, but in the love which asserts the brotherhood of man. The apologists of the first centuries delighted in appealing to the striking fact of the common love of Christians, which was a new thing in the history of mankind; and while the Church has sometimes forgotten the characteristic, the world never has. By their love for each other, for mankind, for God, is it known or denied that men who call themselves Christians are really Christ’s disciples.