Hey,
I did a wedding a few weeks ago. I wasn't the official photographer. I was just testing myself and I listed my findings in the below thread. Go ahead and read them up. You will find some useful info there.
http://www.thephotoforum.com/forum/beyond-basics/195887-my-first-dslr-wedding.html
Your biggest challenge as said will be motion blur. Becase if you use that kit lens and no flash, your shutter speed will not be any faster than 1/30-40 level (depending on the actual lighting) and if the crowd you are shooting is still, you may get away with it but if they are walking or waving their hands, you will get some ugly motion blur. And the funny thing is that you will not realize some of them on that 3.0" LCD screen. This happened to me. When I got home, I had to toss a lot fo good shots due to motion blur.
I did use high ISO 1600 (I have Canon 500D) it was too much for my camera but I had to. So test and find out your max acceptable ISO performance and get ready to use it.
If you have(will have) a prime lens (I used 50mm f/1.4) it will be great. I did captured great shots with that lens inside and outside. I can say that those shots saved me. My friends really liked a few shots I captured and thanked me for those.
But one danger with the fast prime lens is that the shallow DOF. Be VERY careful... One other thing is that they cannot auto focus really fast sometimes and you may lose a good shot...
I cannot explain how stupidly I took many group shots at f/1.4 and found out that all but one person was out of focus in each of those pictures!!!! When taking pictures with prime lens and you stopped all the way up, make sure your subject is in ONE plane or you will burn the picture...
I never used any flash so it was challenging. At the end though I learned alot. If I do another one right now, you bet I would do much better...
A few more things, if you can try to get another camera for the other lens because you will lose some good shots while changing your lenses.
And one last thing, just shoot like crazy maniac. Do not think that you took a good shot and move on. Take 10-15 of the same shots (without annoying people of course) if you can. Because out of 10 shots, you will have much better chance to have a keeper.
Going to church and and doing practice is a very good idea. If it was me right now, I would go to exact same place for the exact time and get myself familiar with the lighting. That way you will now what the bare settings for you...
It's going to be hard I can tell. That's why you should let them know that you will do your best. They shouldn't expect a pro work from you...
Good luck...