John 6:53 - "Except you eat the Flesh of the Son of Man, and drink His Blood, you have no life in you," presents the demand that caused "many of His Disciples to go back, and walk no more with Him" (John 6:66). It does the same presently! This Verse tells us the degree of believing that is required; it refers to the Cross being the total Object of one's belief; failing that, there is no Life in you.
John 6:66 - From that time many of His Disciples went back, and walk no more with Him.
Please note: Jesus called these people Disciples, which means they were saved, but now they are unsaved...they walk no more with Him and have no more life (The Holy Spirit) in them.
This refutes the Unscriptural Doctrine of Unconditional Eternal Security (John 8:31).
JSM