The fact that for some of the disciples needed to eat the flesh and drink of blood of Jesus, to gain eternal live could not be understood by them.

This truely was a radical proposal and unless there had given to each one the ability to accept this by God. Therefore it would be easy to see why some people turned away.

This was one of the main ways the message of Jesus was different to the old Jewish faith.    

This a fundmental principle of belief in the Catholic church and involves the Mass carried out by a "properly" ordained priest who converts bread and wine into His body & blood.

I am told other older break away churches claim the same ability.

This is a fundamental reason to follow a church and not just to pray