John Calvin — Institutes IV, ch. 17, §10 (1559 edition)

“Therefore, if by the breaking of bread the Lord truly represents the partaking of his body, there ought to be no doubt whatever that he truly exhibits and performs it. The rule which the pious ought always to observe is, whenever they see the symbols instituted by the Lord, to think and feel surely persuaded that the truth of the thing signified is also present. For why does the Lord put the symbol of his body into your hands, but just to assure you that you truly partake of him? If this is true let us feel as much assured that the visible sign is given us in seal of an invisible gift as that his body itself is given to us.”