John Calvin Commentary Matthew 26:21

"and as they were eating, he said, Verily I say unto you, that one of you shall betray me." — Matthew 26:21 (ASV)

One of you will betray me. To render the treachery of Judas more detestable, Christ points out its aggravated baseness by this circumstance: that Judas was contemplating the act of betraying him while he sat with him at the holy table. For if a stranger had done this, it would have been more easily endured; but that one of his intimate friends should form such a design, and—what is more—that, after having entered into an infamous bargain, he should be present at the sacred banquet, was incredibly monstrous.

Therefore, Luke uses a connecting particle that marks a contrast: but yet, (πλὴν) lo, the hand of him that betrayeth me. And though Luke adds this saying of Christ after the supper was finished, we cannot determine with certainty from it the order of time, which, we know, was often disregarded by the Evangelists. Yet I do not deny that it is probable that Judas was present when Christ distributed to his disciples the symbols of his flesh and blood.