A.T. Robertson Commentary Mark 14:20

"And he said unto them, [It is] one of the twelve, he that dippeth with me in the dish." — Mark 14:20 (ASV)

One of the twelve (εις των δωδεκα). It is as bad as that. The sign that Jesus gave,

the one dipping in the dish with me (ο εμβαπτομενος μετ' εμου εις το τρυβλιον), escaped the notice of all. Jesus gave the sop to Judas who understood perfectly that Jesus knew his purpose. See on Mt 26:21-24 for further details.