In my denomination (Free Methodist) and a number of other protestant denominations, grape juice is used for communion rather than wine. There is (of course) a longstanding back-and-forth between Christian denominations when it comes to alcohol in general, but the question of wine vs. grape juice in the context of communion is what interests me here.
Specifically, a few years ago I was listening to a radio talk show where the guests were discussing this issue. One of guests stated that John Calvin had once recommended wine as the only appropriate drink for communion, because -- unlike grape juice -- wine combines sweetness and bitterness, which makes a more effective metaphor for the sweetness and bitterness of Jesus' sacrifice on the cross.
I've spent some time trying to search out the original source of this quote but had no luck. Can anyone point me to where it comes from? (Assuming, of course, that the guest on the radio show had his facts straight.)