Church of Simar vs. Cauldron of Blood

I'm owner Cauldron of Blood, must I pay one source when opponent attacks may kingdom zone or I may not pay (and effect don't work)?

You must trigger Forced effect, then: if you have money you you have to pay (church increases the cost of the effect, costs are paid immediately, you can't choose not to trigger Cauldron), if you don't have money effect doesn't work.