John Calvin Commentary John 13:26

"Jesus therefore answereth, He it is, for whom I shall dip the sop, and give it him. So when he had dipped the sop, he taketh and giveth it to Judas, [the son] of Simon Iscariot." — John 13:26 (ASV)

To whom I shall give the dipped sop. It may be asked, what purpose did it serve to give a dipped sop, to discover the traitor, when Christ could have openly pointed Him out by name, if He wished to make him known?

I answer that the sign was of such a nature that it revealed Judas to only one person and did not immediately reveal him to everyone.

But it was advantageous for John to be a witness of this fact, so that he could later reveal it to others at the proper time. Christ intentionally delayed making Judas publicly known so that, when hypocrites are concealed, we may bear it more patiently until they are dragged forth to the light.

We see Judas sitting among the others, and yet condemned by the mouth of the Judge. In no respect is the condition of those who hold a place among the children of God any better.