Realize that every church is gonna be different. I suggest going to the church with what you have- and figure out if you are going to need flash or not. take some shots at f/2.8 - using a stand in subject- shooting at the highest ISO you are comfortable with. With some test shots and knowing that f/1.4 is 2 stops faster than 2.8- you should be able to predict shutter speed with your proposed f/1.4 lens. Double the shutter speed twice of your f/2.8 shots- and you should have about what the f/1.4 lens will net you. If you need to stop down the 50mm f/1.4- well there goes some of that speed. Of course, you will be shooting a much narrower DOF with the f/1.4- so plan that in too.
Of course, you could go the other way too- go with a better body- like a 5D III- which would help more than the f/1.4 lens will. You'd be safer to find out if you can use flash- and use it.
Edit: You should need to get about 1/20th sec for shutter speed using your f/2.8 lens - say at 50mm, to say that the 50mm f/1.4 will be fast enough . If you can hit that- then moving to a 50mm f/1.4 should net you speeds at about 1/80th.. This is based upon crop shutter speed should be about 50mm * crop (1.6) or 1/80th ideally. Without flash to freeze- you probably won't want any slower anyways. I suggested trying it at 50mm- but it really depends upon how you are metering the exposures and which modes you are using.
EOS 5D III, EOS 7D,EOS Rebel T4i, Canon 70-200 f/2.8 IS II, Canon 24-105L, Canon 18-135 IS STM, 1.4x TC III, 2.0x TC III, Σ 50mm f/1.4, Σ 17-50 OS, Σ 70-200 OS, Σ 50-500 OS, Σ 1.4x TC, Σ 2.0x TC, 580EXII(3), Canon SX-40, Canon S100
Fond memories: Rebel T1i, Canon 18-55 IS, Canon 55-250 IS, 18-135 IS (Given to a good home)...