I've shot a couple of church weddings and found that I was fine at around 800 ISO, f/2.8 and 1/100 or thereabouts, without flash. At other services in different venues I've needed 3200 ISO, f/2.8 and 1/60 to get a correct exposure, again without using flash. Whether you would need flash really depends on how the church is lit, what the weather is like and what the insides of the church look like (colour, shape, distance). It also depends on what size prints you want to make and how much noise you are prepared to deal with. I've shot indoor basketball with a 7D at 6400 ISO, no flash. The results weren't awful.
It would probably keep things simpler to avoid flash, so long as the ambient light is of acceptable quantity, quality and direction, but that's the $64,000 question. Without knowing what the church is like, and the lighting on the day, it is hard to give specific advice. If you don't have glass of f/2.8 or faster you might struggle a little, but it really depends.
What I would suggest is that you shoot with manual exposure, so the camera is not fooled by light streaming through windows in the background, or candles and other internal lighting, and also not fooled by dark suits vs pale frocks. Hopefully, the lighting where you shoot will be fairly even and constant, so once you have a good exposure set (check the histogram) then you should be OK for a while.