John Gill 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)

And he answered and said to them
In order to relieve their minds, and point out the particular person:

it is one of the twelve, that dips with me in the dish ;
just at that very instant; (See Gill on Matthew 26:23).