What does Matthew 26:21 mean?

Jesus and His twelve disciples are gathered in the upper room of a house (Matthew 26:17–19). They are all reclining around a low table while sitting on cushions and eating the Passover meal together (Matthew 26:20). At some point during the meal, Jesus makes an announcement that changes the course of the discussion for the rest of the dinner. Jesus declares with absolute certainty that one of these close companions will become a traitor.

Matthew has already revealed that Judas Iscariot has agreed to turn Jesus over to the chief priests for 30 pieces of silver (Matthew 26:14–16). Matthew has not described Judas' other motives. We know from John that Judas has already been stealing from the group moneybag (John 12:6). These perspectives are all hindsight: at the time of this meal, Jesus is the only one who knows which person is false. It will not be until later that Matthew, John, and the rest learn the tragic details.

While Jesus is aware, the rest of the company has no idea that Judas is the betrayer. They are accustomed to hearing difficult pronouncements from Jesus. In fact, they each express fear that they will be the one to fall, despite having no plan to wrong Jesus in any way (Matthew 26:22).