I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear that this happened.
Members of the board of an organization have an obligation to act in the best interests of everyone, and they have an obligation to follow the bylaws. If you donated money to the building fund in good faith, knowing the bylaws required them to return the money to you if the building didn't occur, then they need to return your money because you relied upon those documents.
As long as the donation clearly stated that it was for the building fund, it was a conditional gift. "I am giving you this money as long as you use it to build a new church building." When they decided not to build, the condition on your gift failed, which means it legally must be returned.
You can sue them for violating the bylaws and conversion if they refuse to return your money. Another option is to have a local attorney send them a letter giving them an opportunity to return the funds to you (and any other members who donated for the same purpose), to avoid a lawsuit from all of you. If you can get a few people together to share the costs, that might help, but a demand letter typically only costs a few hundred dollars.
If you have any questions or concerns about my response, please reply WITHOUT RATING. It's important that you are 100% satisfied with my courtesy and professionalism. Otherwise, please rate my service positively so I am paid for the time I spend answering questions. If you are on a mobile device, you may need to scroll to the right. There is no charge for follow-up questions. Thank you.