Augustine of Hippo — as excerpted in the Catena Aurea on John 6:52-54

“(Tr. xxvi. 15) And that they might not understand him to speak of this life, and make that an occasion of striving, He adds, Hath eternal life. This then he hath not who eateth not that flesh, nor drinketh that blood. The temporal life men may have without Him, the eternal they cannot. This is not true of material food. If we do not take that indeed, we shall not live, neither do we live, if we take it: for either disease, or old age, or some accident kills us after all. Whereas this meat and drink, i. e. the Body and Blood of Christ, is such that he that taketh it not hath not life, and he that taketh it hath life, even life eternal.”