Can you carry concealed on someone else's property? Sure you can! Can you do so legally or without getting caught, or in trouble? We aren't lawyers authorized to give you legal advice. I recommend you read State Statutes 941.23(2) and familiarize yourself with it. The question has also been asked multiple times so searches are useful. You have to be careful and precise what your asking and make sure your clear in questions so as to get the best answer. There are links to most of the associated statutes in my New to OC in WI thread. You can get to it here http://Tinyurl.com/OpenCarry-WI or following the link below in my sig. Only you will be held accountable for your actions nobody on the internet or even your lawyer will be able to accept liability for you. So it's best to read and comprehend the statutes for yourself. Hopefully you don't have a thin skin and take any offense to my answer. It really is intended to be in your best interest.
To give you a more opinionated answer to your question. As I read and comprehend the statutes a person can't legally conceal on another's property without the special permission slip to allow an inalienable right.
Who owns this mythical mysterious church? We can't possibly tell you because we don't know which one. Is it owned by a public entity or a private corporation? Most churches are private.