Hilary of Poitiers

(viii. de Trin) Or He means by this, that whereas He was in the Father by the nature of His divinity, and we in Him by means of His birth in the flesh; He on the other hand should be believed to be in us by the mystery of the Sacrament: as He Himself testified above: Whoso eateth My flesh, and drinketh My blood, dwelleth in Me, and I in Him. (supr. 6:54)