Augustine of Hippo — as excerpted in the Catena Aurea on John 6:55-59

“(de Verb. Dom.) As for those, as indeed there are many, who either eat that flesh and drink that blood hypocritically, or, who having eaten, become apostates, do they dwell in Christ, and Christ in them? Nay, but there is a certain mode of eating that flesh, and drinking that blood, in the which he that eateth and drinketh, dwelleth in Christ, and Christ in him.”