18 I do not speak about all of you. I know whom I have chosen; however, so that the Scripture might be fulfilled: “the one eating My bread has raised his heal against Me.”
The disciples may have been bemused by these words of Jesus.
It would appear that they could not imagine that one of them could possible turn against Jesus. They had been through so much together. They had stood loyal to Jesus and to each other. No doubt collectively and individually they were appalled to think that one of their number was about to betray Jesus.
Therefore, they were waiting for the betrayer to be revealed, which was to be done by one of them dipping their bread at the same time as Jesus.
Jesus said this within the context of being servants, and that He had chosen them to be His servants, and the servants of one another.
This is exactly the reason He has chosen us, and every person that He calls.
The salvation that Jesus brings us is the right to be children of God, which entails being His servants, expressed in our serving one another.
Earlier in His ministry Jesus said something quite similar:
“Did not I choose you, and one of you is a devil”? – John 6.70.
Perhaps the disciples were relaxing in the knowledge that Jesus had chosen them. We know that Jesus chose them so that they could be with Him, share His life, and learn His ways of power, authority, and righteousness (Mark 3.14).
Later Jesus was to say something very similar in John 15.16-17:
 “You didn’t choose Me. I chose you. I appointed you to go and produce fruit that will last, so that the Father will give you whatever you ask for, using My name.  I command you to love each other” – (NLT).
It is also true for all who receive Jesus as their Lord and Saviour. He has chosen us. He chose to leave the glory of Heaven and enter this world to pay the price of our sin.
As the song puts it:
You laid aside your majesty Gave up everything for me Suffered at the hands of those You had created You took all my guilt and shame When You died and rose again Now today You reign In heaven and earth exalted
Clearly the betrayer was no longer a servant.