Denominational or nondenominational? Their form of governance, their church constitution, and how they handle the finances in general all need to be taken into account. These things will inform how simple or complex the ability to adopt will be and how best to handle custody.
Discussion
Non denominational, unless Christian union is a denomination. But the church is self governing
Probably best to chat with the elder board then. Might need to give them a pitch on how bitcoin works, why it is good for the church treasury, and discuss self custody.
They would know if there is anything in the constitution or current financial structure that need to be considered or if there is a lot of flexibility for establishing whatever structure they want.