John 6:65

And he said, For this cause have I said unto you, that no man can come unto me, except it be given him of the Father.

See under John 6:44-45 for comment on how God draws men, and how he gives men power to come to Christ. The Lord's evident purpose here was to encourage the faithful disciples. The defection of many, the unbelief of some, and the treachery of one - all of these were events which were due to the fact of their rejecting God's word by which they would have been drawn to Jesus, and thus it was not given unto them by the Father to be Jesus' disciples and to have eternal life.