"Doubtless when Jesus washed the disciples' feet he included the feet of Judas Iscariot. If this proves anything beyond the unfathomable love and forbearance of the Master, it is that no rite, even if performed by Jesus himself, ensures spiritual cleansing. Washed Judas may have been; cleansed he was not (cf. 6:63-64). The only other place in the Fourth Gospel where Jesus tell his disciples (minus Judas) that they are clean is 15:3: 'You are already clean because of the word I have spoken to you.' Real cleansing is effected both through Jesus' revelatory word and through the atoning sacrifice to which the footwashing pointed."From The Gospel According to John: The Pillar New Testament Commentary, D. A. Carson , Grand Rapids, MI: Wm. B. Eerdman's Co., p. 466
In his commentary, Carson also points out that there are three applications of the feet washing"1. The impending atoning death of Christ on the cross, vv 6-82. The cleansing of the progressive Christian experience in time, vv 9-11 cf v 10 with I John 1:93. The lesson of love and humility ,vv 12-17
According to John Christopher Thomas, there is no historical record in either Jewish or Greco-Roman sources "of a superior washing the feet of an inferior."
Footwashing in John 13 and the Johannine Community, John Christopher Thomas, Sheffield, JSOT Press, 1991, Chapter 3
To the extent that we want power we are in the flesh, and the Holy Spirit has no part in us. Christ put a towel around Himself and washed His disciples' feet (John 13:4). We should ask ourselves from time to time, "Whose feet am I washing?" Some churches have made foot-washing into a third sacrament; members wash each other's feet during their worship service. While most of us think it is a mistake to make this a sacrament, let us admit that it is 10,000 times better to wash each other’s feet in a literal way than never to wash anybody’s feet in any way. It would be far better for us to make a mistake and institute a third sacrament of literal foot-washing than to live out our lives without once consciously choosing to serve each other. Doing the Lord’s work in the Lord’s way is not some exotic thing; it is having and practicing the mentality which Christ commands.From The Complete Works of Francis A. Schaeffer: a Christian Worldview. Volume Three, a Christian View of Spirituality, Francis August Schaeffer, Westchester, IL: Crossway Books, 1982, pp. 46-47