“But this is to fulfill this passage of Scripture-”
Jesus was calling attention to the fact that this event had already been written about and was about to be fulfilled. Telling them about it should have bolstered their faith.
The fact that they didn’t know who the betrayer was is interesting. Evidently, Judas had not acted strangely to them. We know that he used to help himself to the kitty every so often (12:6), but he must not have shown any obvious discontent. Maybe it was all money-related. Whatever the motivation, he gave in to temptation.
13:7 is VERY interesting, “As soon as Judas took the bread, Satan entered into him.” The verse is not enough on which to build an entire doctrine, but it does give us a window into the spirit world. “Intent.” It could be that we give permission to demons to take control-to a limited degree-when we decide to sin. In reverse, we give permission to the Holy Spirit to do the same when we decide to obey the Lord.
Let me know your thoughts on this matter.