St. Thomas Aquinas
“Hilary says (De Trin. viii): ‘There is no room for doubt regarding the truth of Christ’s body and blood; for now by our Lord’s own declaring and by our faith His flesh is truly food, and His blood is truly drink.’ And Ambrose says (De Sacram. vi): ‘As the Lord Jesus Christ is God’s true Son so is it Christ’s true flesh which we take, and His true blood which we drink.'”
Source: St. Thomas Aquinas, Summa Theologica