I don't really recommend newbies shooting wedding with available light, these are the following pitfalls;
inconsistent white balance, give extra work for post production,
unable to handle situation in back lighting,
unable to see unwanted highlight/shadows appear in frame,
poor camera handling in slow shutter speed,
if you don't have the knowledge, skills, hardware, and software in photography and post production to handle all the issues,
it is better to get a flash, keep in simple, give your couple sharp sharp clear clear photos.