Under Charlemagne in the 8th century and the Carolingian emperors, Jews were official importers of exotic foods from the Holy Land (source: https://books.google.com/books?id=GkzdBDuhoRgC&pg=PA120&q=exotic%20foods); they also provided wine for Christian mass.
What were the halachic issues involved? Food issues? Basar b'chalav? Is it forbidden, in general, to provide wine for use in a Christian religious ceremony?
And, in the end, was it halachically allowed or forbidden for the Jews to provide the wine?