Origen
Catena Aurea · Johannesevangelium 13
So after he had washed their feet, and had taken his garments, and was set down again, he said unto them, Know ye what I have done to you? Ye call me Master and Lord: and ye say well; for so I am. If I then, your Lord and Master, have washed your feet; ye also ought to wash one another’s feet. For I have given you an example, that ye should do as I have done to you. Verily, verily, I say unto you, The servant is not greater than his lord: neither he that is sent greater than he that sent him. If ye know these things, happy are ye if ye do them. I speak not of you all: I know whom I have chosen: but that the Scripture may be fulfilled, He that eateth bread with me hath lifted up his heel against me. Now I tell you before it come, that, when it is come to pass, ye may believe that I am he. Verily, verily, I say unto you, He that receiveth whomsoever I send receiveth me; and he that receiveth me receiveth him that sent me. If then I your Lord and Master have washed your feet, ye also ought to wash one another’s feet.
(t. xxxii. 8.) Or thus: I speak not of you all, does not refer to, Happy are ye if ye do them. For of Judas, or any other person, it may be said, Happy is he if he do them. The words refer to the sentence above, The servant is not greater than his lord, neither He that is sent greater than He that sent Him. For Judas, being a servant of sin, was not a servant of the Divine Word; nor an Apostle, when the devil had entered into him. Our Lord knew those who were His, and did not know who were not His, and therefore says, not, I know all present, but, I know whom I have chosen, i. e. I know My Elect.