As quoted at the end of J.W. Nevin’s The Mystical Presence, pp. 314:
“They are preposterous,” says Calvin, “who allow in this matter nothing more, than they have been able to reach with the measure of their understanding. When they deny that the flesh and blood of Christ are exhibited to us in the Holy Supper, Define the mode, they say, or you will not convince us. But as for myself, I am filled with amazement at the greatness of the mystery. Nor am I ashamed, with Paul, to confess in admiration my own ignorance. For how much better is that, than to extenuate with my carnal sense what the apostle pronounces a high mystery!