Ambrose, Bishop of Milan in one of the early centuries of the church, believed that the washing of feet instituted by Jesus was as sacramental, as important and as necessary as the two sacraments that the churches have all ended up accepting as those given us directly by Jesus’ word and action:
Baptism, accepted by Jesus and commanded at the end of Matthew’s Gospel: Go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit; Eucharist, rehearsed by Jesus at the Last Supper, as we celebrate tonight, and commanded to continue in his memory.
But look, Ambrose might say, at what the Gospel says:
Jesus, knowing that the Father had given all things into his hands, and that he had come from God and was going to God, rose from supper, laid aside his garments, and girded himself with a towel. Then he poured water into a basin, and began to wash the disciples’ feet, and to wipe them with the towel with which he was girded. When Peter demurred, Jesus pressed him, saying, “If I do not wash you, you have no part in me.” Simon Peter said to him, “Lord, not only my feet but also my hands and head!” When he was done, and had resumed his place, Jesus said to them, “If I have washed your feet, you also ought to wash one another’s feet. For I have given you an example, that you also should do as I have done to you.”
It amuses some liturgical wits to wonder then whether we might not as easily have found ourselves celebrating the washing of one another’s feet as the central sacrament of our Sunday worship, with the Eucharist reserved for the annual commemoration of the Last Supper on Maundy Thursday.
But we read the washing of feet as Jesus’ signpost to his disciples of what was to come, his foreshadowing of his offering of himself, body and spirit, on the cross for our salvation, for the forgiveness of sins and for the reconciliation of the world to its God, his demonstration of love poured out unconditionally, freely and wholly, for all people.
Just as Mary had anointed the feet of Jesus, and wiped them with her hair, pouring out her love and her grief and her hope, her sorrow at his leaving, her hope for his return, so Jesus bids farewell to his disciples in a tender gesture, offering them his humble thanks for their friendship, his promise that he would do anything for them, his example of deep and active love.
It is so difficult to let someone love us. It is so difficult to receive a gift with no means of return, to receive a blessing, forgiveness for something we can barely forgive ourselves, the admiration of one whom we admire, or despise. We want to stay in control, to do for ourselves, to owe no one anything.
But we cannot control God. There is nothing that we can offer to God which has not been provided to us by God’s grace. Our own selves, our souls and bodies are creations which we give back only to the one who gifted them to us.
As Jesus washed the feet of his disciples, as tender as a mother, as humble as a slave, as intimate as a woman, as Mary of Bethany had been with him, he demonstrated the all-encompassing, abounding love of God for each of them, for all of them.
As he shared with them a meal baring his body and soul, his blood which would be poured out for them, his love with which he laid down his life for them, for us, he showed them the lengths that God will go to simply to be with us.
A sacrament, as you know, is an outward and visible sign of God’s inward and invisible grace. What Jesus offered his disciples was unmistakable. Then he said, “You also should do for one another what I have done for you,” that is, be outward and visible signs of God’s grace, witness to God’s evident love for all of God’s creation, and be ready to receive that love ourselves, without condition or control, without holding back, but like Peter diving headlong into the gift of God’s mercy.
“Love one another,” said Jesus, “as I have loved you.”
 St Ambrose, Bishop of Milan d.397, De Sacramentis (On the Sacraments), Book 3, iv-vi; source: Documents of the Baptismal Liturgy, E.C. Whitaker, rev. and exp. By Maxwell E. Johnson (Collegeville, Minnesota: Liturgical Press, third edn 2003), p. 180