John Calvin Commentary John 13:35

"By this shall all men know that ye are my disciples, if ye have love one to another." — John 13:35 (ASV)

By this all men will know. Christ again confirms what he previously said: that those who mutually love one another have not been taught in his school in vain. It is as if he had said, “Not only will you know that you are my disciples, but your profession will also be acknowledged by others to be sincere.”

Since Christ sets forth this mark for distinguishing between his disciples and strangers, those who set aside brotherly love and adopt new and invented modes of worship labor in vain; and this kind of folly prevails today in Popery.

Nor is it superfluous that Christ dwells at such length on this subject. There is no greater agreement between self-love and the love of our neighbor than there is between fire and water. Self-love binds all our senses in such a way that brotherly love is entirely banished. Yet we think that we fully fulfill our duty, because Satan has many enticements to deceive us so that we do not perceive our faults.

Therefore, whoever desires to be truly a disciple of Christ and to be acknowledged by God should shape and direct his whole life to love the brethren, and pursue this goal with diligence.