54 do non-Catholics, non-Christians not understand? And, why? Christ said this sentence and highlighted it with "Amen, amen", which means "pay attention, please".
Life in this verse means sanctifying grace...no Eucharist, no grace, no salvation, no heaven.
Then Jesus said to them: Amen, amen I say unto you: Except you eat the flesh of the Son of man, and drink his blood, you shall not have life in you.