John 13:15

For I have given you an example
Christ is an example to his people, in many things; not in his miraculous performances and mediatorial work, but in the exercise of grace, of meekness, humility, love, patience, and the like; and in the discharge of duty, in submission to ordinances, and in attending on them; and in the several duties, both to them that are without, and to them that are within; and also in his sufferings and death; not that he died merely as an example, but likewise in the room and stead of his people; but here he is spoken of, as an example, in a particular instance:

that ye should do as I have done to you;
wash one another's feet, as he had washed theirs; which is not to be understood literally and singly of this action, as though this was an ordinance binding upon all persons, in all places, and to be attended to at certain stated times, as has been the practice of some: it was so understood by the church at Milain, and there practised; and this custom was continued and defended by St. Ambrose, even though not received by the church of Rome; in some places the bishop used to wash the feet of those that were baptized, which in process of time being thought sufficient, instead of baptism, was forbidden by the council at Eliberis. In imitation of this, the pope every year, on Thursday in the passion week, washes the feet of twelve men; and it is an anniversary ceremony performed by the kings of England and France, to wash the feet of twelve poor people, in commemoration of this action of Christ's: but our Lord is not to be understood literally, nor of anything that was to be done once a year, but of what was daily and constantly to be practised; and which was to be done not by one only, to all the rest, but what they were mutually to do; what they were to do to one another; for the thing signified, reaches to, and is obligatory upon all Christians. Our Lord's meaning is, that as he had, by this action, given them an example of humility, condescension, and love; so they should exercise these graces, and perform such kind offices to one another, and to all their fellow Christians.