William Burkitt on John 6:64

Our blessed Saviour having thus cleared his doctrine, that he was the bread of life which came down from heaven, and that he is not to be carnally, but spiritually, fed upon: he plainly tells the Jews, That the true cause of their stumbling at his doctrine, was their ignorance and unbelief; There are some of you that believe not. Upon which plain dealing of our blessed Saviour's, many unsound professors did wholly forsake him, and accompanied no longer with him.

Learn hence, That multitudes, who have long professed Christ and his holy religion, may draw back, and fall from their profession, and finally revolt from him.

2. That it is an evil heart of unbelief, which causes men to depart from Christ, and to make shipwreck of their profession.