Kenneth N. Taylor, in his Living Gospels: The Paraphrased Gospels, puts it this way: "Master, You shouldn't be washing our feet like this!" Christ responds, "What I am doing you do not understand now, but you will know after this" (verse 7).
Peter, still not convinced, flatly states, "You shall never wash my feet" (verse 8). Christ's next words, however, finally cause him to give in: "If I do not wash you, you have no part with Me." Jesus' response—whatever His tone of voice—struck the disciple squarely between the eyes: Peter's eternal life was on the line! This time, his response is quite different: "Lord, not my feet only, but also my hands and my head!" (verse 9). This sounds a little like Psalm 51:2, where David prays, "Wash me thoroughly. . . ."
Our Savior's answer to this request is not what some would expect: "He who is bathed needs only to wash his feet, but is completely clean" (John 13:10). The New International Version's translation of this verse makes His thought clear: "A person who has had a bath needs only to wash his feet; his whole body is clean." Obviously, the disciples had bathed before coming to eat the Passover meal. But having to walk along dusty roads in sandals, they collected a small amount of dirt on their feet. Thus, Christ explains that to be perfectly clean again, all He needed to do was to wash their feet.
Upon repentance, baptism, and receiving God's Holy Spirit, we are at that point perfectly clean in God's eyes. The blood of Jesus Christ has symbolically washed away all our past sins, and we stand before Him completely sinless. We have been buried in the waters of baptism and resurrected to a new life. However, as we all know, our human nature has certainly not departed from us, and it is not very long until the fact that we have sinned again stares us in the face. The old self has not really gone away; our lives are much as they were—with some important exceptions: We now have the Holy Spirit and have been given God's grace.
Having been regenerated by God through His Spirit that He has given us, we have entered a unique relationship with God the Father. The veil that once separated us from having access to Him has been torn away by the death, resurrection, and ascension of Jesus Christ (Matthew 27:51; Hebrews 6:19-20; 10:19-22). Through Him, we can communicate with the Father to seek mercy and forgiveness for our sins and weaknesses. Upon repentance, God applies again the sacrifice of Christ to us and forgives us by His grace.
Now we can see that, even though we were once washed completely clean at baptism, we will occasionally sin as we walk through this life. We will spiritually get our feet dirty, and we will need Christ to wash our feet to make us completely clean again. Thus, He tells Peter, if He did not wash his feet, he would have no part with Him. None of us can carry unforgiven sins and still remain part of the body of Christ. This points out why it is so imperative that we seek His mercy and help to repent each day. When we do this, He can symbolically wash our feet and make us clean again. Each year at the Passover service, we reenact this to remind us how important it is.