I too would suggest something like CushyCMS as mentioned above already. This would allow you to give access to specific people in specific areas of the site (content only) so that it doesn't get too messed up. Each person could have their own account for whatever department of the church they are responsible for updating the site for. You can then specify areas of the site they have control of and you can even make it only the content area of a page.
I wouldn't suggest full FTP control for multiple people. I think that would be a bit overkill. Control should be given to at least one other person in the church, but should be given to someone that has good judgement and is responsible enough not to cave in and give it to everyone that wants it. In most cases this is the person in charge of dealing with getting the website taken care of (hired or asked you to work on it). For the rest I'd only give them access to a single folder where they can upload text files for you to put their content into the site. Wouldn't let them do it to be honest.
Should they get very insistent and require you to hand over the info so they can work on it to, then tell them a price (and not a cheap one, no less than $30.00 per hour) for fixing it if they mess it up. Being a VSD site they will be required to purchase VSD for EACH person that will be involved in it. And as already mentioned the mess will be in whether they forget to download the .vnu file from the server and open that one instead of the one they had already saved themselves last time they worked on it. This means nasty overwrites of other people's work etc. It will be a mess. It's hard enough with only 2 people trying to work with VSD sites let alone a group.
Make sure those terms are clear if they insist on it and DO NOT let them trample you just because they are a church. That doesn't give them the right to abuse your time and efforts and most importantly your contribution of doing a free site for them. They've already abused you in condemning you for not using open source software which really is no concern of theirs what you choose to use if they aren't paying for it lol.
Just my 10 cents