R' Moshe Feinstein's Catholic Israel

From here

Rav Moshe Feinstein ruled that Catholics are allowed to use grape juice for the eucharist. To be more precise, R' Dr Aaron Twersky is both a rabbi (from a long line of Chassidic Rebbeim, but I mean "rabbi" in the sense of having a synagogue, not the head of a community) and a psychologist who works in a substance abuse center.

One of his patients was a preist who was being cured of alcoholism. This means he can't drink even a drop of wine again; a problem for a priest who must take communion. His self-control just isn't reliable. So what is he supposed to do during Mass? He voiced this concern to Rabbi Twersky, who asked why he didn't just use grape juice.

The priest asked his cardinal, and eventually the problem made it all the way to Rome. Can grape juice be used for communion? Well, the Vatican heard that we consider the grape juice a kind of wine, but they wanted to know more. So the question went back to the priest and Rabbi Twersky. R' Twersky sent back a citation of a responsum of R' Moshe Feinstein, allowing the use of grape juice for the seder. The Vatican concluded that if grape juice is okay for the seder, then it was usable for the Last Supper, and therefore when Jesus said at that meal "this is my blood" he meant grape juice too.