My understanding is that one is not aloud to intentionally sell anything to an oved avodah zarah ("idol worshipper") if he knows that he will use that item for his idol worshipping purpose.
There are two "mitzvah items" that I have seen Christians use as part of their Church service, and regarding each of these, I'm curious if this prohibition applies:
- Can one sell pieces of his lulav to a Christian for him to use as part of Palm Sunday services?
- Can one sell matzot shel mitzvah (i.e. - those specially baked for Pesach Seder use) to a Christian who uses them as the "host" as part of mass?
Factors to include in your answer, if applicable include:
- any halachot of Jews selling items to Gentiles for their worship purposes
- any special kedusha (holiness) status to left-over lulavim or matzot after the holidays
- whether today's Christians are in the category of aku"m (idol worshippers) even if they do attend a service, occasionally.
