Obviously it would have been better to check out the venue ahead of time to be able to get prepared, especially if you're doing weddings or paid photography for clients. Unfortunately I was thinking along the same lines as John (tirediron) and thinking he was probably on the right track, but if it's too late to make other arrangements...
I do sports/events in existing light (but not weddings) and can manage in low light, but it can have its limitations in what can be done, it's just pushing the limits of what a camera can do. For example I can get a shot in a spotlight in a dark arena, and have done an occasional candlelight shot. But I think it takes learning to be able to do that type thing.
If you have to use the existing light I'd use a sharp lens and open it up to let in as much light as possible, but I don't know if you'll get really sharp great quality photos if the light is really dim. If it's hard to focus find a sharp edge of an object to focus on and reframe as needed.
I'd try to notice where the lights are and get positioned under them, or try to get the subjects as they step into or stand under the lights. Walk around ahead of time and do some test shots and see where (from what angle or position)s you think you could get the best photos so you know where to be during various parts of the ceremony.