In John 13: 2-17 is the well-known story of Jesus washing his disciples feet. At the end of this sermon-by-demonstration, Jesus tells the disciples that he has given them an example that they should follow. Most teachers I have heard understand Jesus to be saying that we should serve one another in all sorts of ways, not just by washing feet.
All well and good. I agree.
But as I read the passage recently, I suddenly noticed in the middle of the story an interesting little sermon-within-a-sermon. Jesus replied to Peter - who had in his impetuous Peter-like way insisted that Jesus wash all of him, not just his feet - that, "A person who has had a bath needs only to wash his feet; his whole body is clean. And you are clean, though not every one of you."
As is so often the case, Jesus takes an exterior example to make an interior point. If Jesus was talking about dirt on the body, it seems unlikely he would have excluded Judas from the ranks of the clean. And this seems especially unlikely since John specifically indicates (13:11) that the reason one of the disciples (Judas) was not clean was because he would betray Jesus. So it seems quite clear that Jesus is saying that his disciples (except Judas) were spiritually clean, and - if I may extrapolate a bit - fit for heaven.
But, if they were clean, then why did Jesus need to wash their feet?
I think Jesus meant that though they were clean and devoted to God, just walking around in this dirty world tends to stain them (and us) with various sins. We say and do bad things on the spur of the moment, or in a flash of passion or anger or inattention or weariness, things that smudge us with the dirt of sin. And though we are basically clean inside - we really do love Jesus and we really are citizens of heaven - we still need to have Jesus wash away those daily sins.