Here are a few thoughts....

First, I like the idea of having a network people can use. It makes the church more usable.

That being said, I think this is something that needs to be done right. Separate the open wifi from the church network. You shouldn't be able to access any of the churches computers, printers, or network stuff from the open wifi. If you want that access add a second wifi with security and only let the church staff have access it.

The separation can be done by having the two networks connect directly to your line out (cable connection or what ever) and create 2 distinct church networks.

On the open network have an agreement acceptance page. Kind of like what you get when you go to Panera Bread and use their wifi. It's free but you want to cover your rear.

If 'bad' material is an issue you can put a filter on the open network. Though, I would recommend against it unless you have a good reason. There is something that comes out when someone hits that filtering in the way they look at you for not trusting them and they can hit those messages on false positives. For example, I know a great technical blog that get's blocked for it's material when there is nothing on there but really geekie stuff. It's a false positive.

You might, also, want to consider a proxy server or limiting the bandwidth the open connection can use so someone on it doesn't use up all the network bandwidth and kill productivity in the office.