There are three big problems with this sale.
1) A shul was disgraced by its use as a house of Idol Worship!
2) The sellers aided the Idol Worshippers by providing a beautiful place to practice their Avodah Zara.
3) How can the one who sold it say “Whatever proceeds there are will go to continue [our] mission” ? The proceeds of the sale are tainted by benefitting from Avodah Zara!