Feet Washing

John 13:10 (KJV)
Jesus saith to him, He that is washed needeth not save to wash his feet, but is clean every whit: and ye are clean, but not all.

The feet washing account can be taken at various levels.

The main thrust of Jesus' message is that of servitude (v. 15).  That even the Son of God came not to be served but to serve, to give his life a ransom for many (Mat 20:28).
Wherefore, we should similarly serve one another and our fellow man.
Why is it not to be taken literally? Because the context is not about the physical, as shown here in the verse but is symbol. Additionally, we do not read of feet washing being practiced throughout the rest of the New Testament

In order to be part of Christ, we have to obey what he commands. No more, no less. The ever impetuous Peter immediately tried to "outdo" himself by counter proposing, but as we all know, one cannot make one "more righteous" by doing beyond what is commanded.

On a more "speculative" level, if you please, is the idea of the state of mind when one truly obeys the gospel. One of the criticisms of a "plan" of salvation is the feeling of having form without function. Almost like the attitude of the Pharisees. Some have even extended to all forms of organised worship acts. An most certainly, one can merely go through the motions of worship, even the steps to salvation without genuine conviction. The contrast here is given between Judas and the rest. The others could he made "clean" because of their purity of desire. Judas on the other hand, had already made up his mind to betray Jesus, hence can never be made clean. In a like manner, even as we learn of the steps to salvation, yes, one can be like clean Peter, to truly believe by showing genuine repentance and the commitment thereof to make right, thereby making sense of baptism. But an unclean Judas attitude is one that makes the steps mere ceremony and does nothing for the soul.
Nevetheless, it remains for us to learn and obey what God wants us to do, and do it whole heartedly, in all purity. That is exactly what Jesus said of "sprit" and "truth" (Jhn 4:24). And we need not be stuck in the stage of unbelief, but as and when we truly turn and obey, God's grace and mercy will be extended.
Judas was given chances to repent, even after he realised his folly. Yet he blew them away by again taking things into his own hands, and taking his own life.