In my opinion, you're going to struggle with image-blur with only f/2.8 in a dark church without the help of IS or a flash. What camera will you be using it on? I've used an 85mm at f/2 plenty of times at iso1600 in really dark situations on a 20D. Then again, that's a stop faster, and on the short end of your focal length range. How do your images look at high-iso's on your camera? If a flash is allowed, use it.
Also, you may be better off investing in better wide glass. It depends on your shooting style, but I would personally find the kit lens limiting, and you can always move closer during the ceremony, but you can't always back up during the reception.
Other people will have other opinions and say a 70-200 is a necessity, but I think most of those people would agree that something faster and wider would also be necessary.