John Gill Commentary John 6:71

"Now he spake of Judas [the son] of Simon Iscariot, for he it was that should betray him, [being] one of the twelve." — John 6:71 (ASV)

He spoke of Judas Iscariot, [the son] of Simon
These are the words of the evangelist, pointing out the person Christ intended, lest any other should be suspected:

for he it was that should betray him :
as it was determined and foretold, and which Christ knew full well, and therefore said the above words:

being one of the twelve ;
apostles, whom Christ had chosen, and which was an aggravation of his crime.