John 6:66

Charles Haddon Spurgeon, preaching on the passage from John 6:66, “From that time many of his disciples went back, and walked no more with him” said, The defection in this case was on account of doctrine... The truth was too hard for them, it was not to be borne with. “It is a hard saying. Who can hear it?” A true disciple sits at the feet of his Master, and believes what he is told even when he cannot quite comprehend the meaning, or see the reasons for what his Master utters; but these men had not the essential spirit of a disciple, and consequently when their Instructor began to unfold the innermost parts of the roll of truth, they would not listen to His reading of it. They would believe as far as they could understand, but when they could not comprehend they turned on their heel and left the school of the Great Teacher. Besides, the Lord Jesus Christ had taught the doctrine of the sovereignty of God, and of the need of the Spirit of God, that men should be led to Him, “for Jesus knew from the beginning who they were that believed not, and who should betray him. And he said, Therefore said I unto you, that no man can come unto me, except it were given unto him of my Father.” Here our Lord uttered a bit of old-fashioned free-grace doctrine, such as people nowadays do not like. They call it “Calvinism”, and put it aside among the old exploded tenets which this enlightened age knows nothing of. What right they have to ascribe to the Genevan reformer a doctrine as old as the hills I do not know. But our Lord Jesus never hesitated to fling that truth into the face of His enemies. He told them, “Ye believe not because ye are not of my sheep, as I said unto you.” “No man can come to me, except the Father which hath sent me draw him.” Here he tells them plainly that they could not come unto Him unless the Father gave them the grace to come. This humbling doctrine they could not receive, and so they went aside. (CHS, Sermons, 28, 111-2)

taking from http://vintage.aomin.org/doctrineofman.html