For he knew who was going to betray him, and that was why he said not every one was clean.
Jesus had known for at least a year that Judas was going to betray Him, and yet He treated him no differently than the others (6:64,70). He didn’t even tell the others who the traitor was. Robertson wonders, “Did Judas wince at this thrust from Jesus?”
John 6:64 Yet there are some of you who do not believe.” For Jesus had known from the beginning which of them did not believe and who would betray him.
John 6:70 Then Jesus replied, “Have I not chosen you, the Twelve? Yet one of you is a devil!”