John 13:18

We have here the discovery of Judas's plot to betray his Master. Christ knew it from the beginning; but now first he discovered it to his disciples, who did not expect Christ should be betrayed, though he had often told them so, much less did they suspect that one of them should do it. Now here, I. Christ gives them a general intimation of it (Joh 13:18): I speak not of you all, I cannot expect you will all do these things, for I know whom I have chosen, and whom I have passed by; but the scripture will be fulfilled (Psa 41:9), He that eateth bread with me hath lifted up his heel against me. He does not yet speak out, either of the crime or the criminal, but raises their expectations of a further discovery. 1. He intimates to them that they were not all right. He had said (Joh 13:10), You are clean, but not all. So here, I speak not of you all. Note, What is said of the excellencies of Christ's disciples cannot be said of all that are called so. The word of Christ is a distinguishing word, which separates between cattle and cattle, and will distinguish thousands into hell who flattered themselves with hopes that they were going to heaven. I speak not of you all; you my disciples and followers.

— Matthew Henry's Concise Commentary (public domain)

I speak not of you all--the "happy are ye," of Joh 13:17, being on no supposition applicable to Judas. I know whom I have chosen--in the higher sense. But that the scripture may be fulfilled--that is, one has been added to your number, by no accident or mistake, who is none of Mine, but just that he might fulfil his predicted destiny. He that eateth bread with me--"did eat of my bread" (Psa 41:9), as one of My family; admitted to the nearest familiarity of discipleship and of social life. hath lifted up his heel against me--turned upon Me, adding insult to injury. (Compare Heb 10:29). In the Psalm the immediate reference is to Ahithophel's treachery against David (2Sa. 17:1-23), one of those scenes in which the parallel of his story with that of His great Antitype is exceedingly striking. "The eating bread derives a fearful meaning from the participation in the sacramental supper, a meaning which must be applied for ever to all unworthy communicants, as well as to all betrayers of Christ who eat the bread of His Church" (STIER, with whom, and others, we agree in thinking that Judas partook of the Lord's Supper).

— Jamieson-Fausset-Brown Commentary (public domain)