John Calvin on Matthew 26:23

23. But he answering said. Christ, by his reply, neither removes their doubt, nor points out the person of Judas, but only confirms what he said a little before, that one of his friends sitting at the table is the traitor. And though they thought it hard to be left in suspense and perplexity for a time, that they might employ themselves in contemplating the atrocity of the crime, it was afterwards followed by another advantage, when they perceived that the prediction of the psalm was fulfilled,

He that ate pleasant food with me “Celuy qui mangeoit en ami avec moy;” — “he that ate with me as a friend.”

hath lifted up his heel against me, (Psalm 41:10.)

Besides, in the person of Judas, our Lord intended to admonish his followers in all ages, not to be discouraged or faint on account of intimate friends proving to be traitors; because the same thing that was experienced by Him who is the Head of the whole Church, must happen to us who are members of it.