Jesus disciples couldn't really comprehend the fact that Jesus was going to die soon, they thought He was going to be a great military leader who would soon show His might against their Roman conquerors. They wondered who among them would be His highest officers. Jesus let them know His kingdom was different than that of this world, He taught them to be great in His kingdom they must be willing to be a servant of all, they must be willing to be as humble as a little child.
Jesus, because He loved His disciples, and knowing He would soon be put to death, now would teach them a lesson with practical as well as eternal implications. He took off His garments and put them aside and wrapped a towel around Himself. He poured water into a basin and began to wash His disciples feet. They were horrified. This was the Master, their teacher, their Lord, and He was doing only what the lowest of servants would be ordered to do. They had walked the unsanitary, vermin infested roads, covered with animal feces, and dead carcases. Their feet were filthy and the Master was going to wash them. NO, Peter objected, I will never allow You, my Lord and Master, to wash my feet.
Jesus loved the brash, outspoken Peter, but He let him know authoritatively that if Peter would not allow Him to wash his feet, Peter would have no part with Him. This was a startling revelation to Peter, then wash my hands and head also he pleaded. They had already bathed in preparation for the meal, they were clean except for their feet which had gotten dirty on their way there. Jesus told Peter there was no reason to wash what was already clean, only that which had gotten dirty. He then taught them a lesson about greatness and humility, if He as their Lord and Master would take on the responsibilities of a servant, if they wanted to be great it began by being willing to serve.
As Christians we're clean, but we live in a world that is not clean, some of the ideas and immoral attitudes of our secular world try to rub off on us. If we succumb we need only to come to the Father and confess our sins. 1 John 1:9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.