Jesus knew who was going to hand him over to his enemies. That was why he said not every one was clean.
When Jesus finished washing their feet, he put on his clothes. Then he returned to his place. "Do you understand what I have done for you?" he asked them.
"You call me 'Teacher' and 'Lord.' You are right. That is what I am.
I, your Lord and Teacher, have washed your feet. So you also should wash one another's feet.
I have given you an example. You should do as I have done for you.
"What I'm about to tell you is true. A servant is not more important than his master. And a messenger is not more important than the one who sends him.
Now you know these things. So you will be blessed if you do them.
"I am not talking about all of you. I know those I have chosen. But this will happen so that Scripture will come true. It says, 'The one who shares my bread has deserted me.'(Psalm 41:9)
References for John 13:18
"I am telling you now, before it happens. When it does happen, you will believe that I am he.
What I'm about to tell you is true. Anyone who accepts someone I send accepts me. And anyone who accepts me accepts the One who sent me."
After he had said this, Jesus' spirit was troubled. Here is the witness he gave. "What I'm about to tell you is true," he said. "One of you is going to hand me over to my enemies."