Here we see the confusing part of the passage. We are left wondering on our first reading why no one understood that Judas was going to betray Jesus. Had God clouded their minds? It just seemed odd and confusing. However, when we see that Peter had made the motion to John to speak and that what we have seen was a private conversation between Jesus and John it makes sense.
We also see how sneaky and sly the wicked can be. Jesus has known for some time that Judas was going to betray him. However, the disciples seem to be unaware of Judas’ treachery. When Jesus announced that one of them would betray Him they all looked around at each other. They did not all look at Judas. He seemed to be just as committed as the rest of the disciples, yet he had been a thief and a traitor all along.
I think we must be careful to examine everyone. We must realize the power and depth of sin in people's lives. We must not be surprised when people show themselves to not be Christians even though they have been in the church.
Consider: 1 Peter 4:12-15