That's a good point that I forgot to mention.

When shooting with ambient light, it is often necessary to boost your ISO, but to keep the graining or noise out of your shot, you really do want to shoot at as low an ISO as you possibly can. So by reducing the ISO, you will need to add light from another source, which is your flash.

Again, you need to experiment with your settings under a number of lighting situations.

And remember that time of day, angle of the sun, time of year, and clouds will all effect your indoor shots with ambient light, and the amount of fill flash you'll need.

Even shooting outdoors, often a fill flash comes in handy to bring out the highlights of a shot. I once shot a GBH (Great Blue Heron) where I used my fill flash at a very high angle, just to bounce some light off of the greenery behind the bird, and it absolutely made the bird pop out of the picture with details.

As I said earlier, your fill flash will quickly become your best friend in many situations. Often, I'll keep it on my camera, ready to use if I need it, but I don't turn it on unless I really do need it. Yes, it's added weight, but there are times when you just don't have time to dig it out and attach it before you lose your subject.

At 1/30, when shooting a group, your shutter speed would be too slow.

Using a bounce or fill flash at a reduced output, and about a 45 degree angle would add some light, but not overpower the existing light and colors. Flashes work best at shutter speeds between 1/60 & 1/250 with Canon Cameras.

Ask for permission to go into the church at about the time of day you would need to shoot there in the future, and try shooting a live subject at the alter with the ambient lighting, then adding the fill flash at a 45 degree angle try different settings both on the flash, as well as the shutter speed, f-stop, and ISO. It's going to be a matter of experimenting until you get close to what you're looking for.

Each time you shoot in a church, you may need to try some experimental shots before the services begin, to check the available lighting, and what to use for settings to work around the around available lighting.

Be aware of clouds, and their potential for blocking the sunlight, which will severely effect your shots.