I think your issue here is two-fold. You're not just trying to capture fast action, you're trying to capture fast action in low light.

There is no hard-and-fast set number that will guarantee you won't have any motion blur from fast action. The faster the action, the faster your shutter speed needs to be to "freeze" it.

The problem that's probably happening is that indoors, in a church, your light levels are low enough that even with a high ISO setting and your lens's maximum aperture setting, you still need a slower shutter speed to get a good exposure.

To get faster shutter speeds, you can only do a few things, if you're already maxed out on iso and aperture.

• You can get a faster lens. Many consumer-grade lenses max out at f/5.6. This isn't particularly wide, and is called a "slow" lens, because of the shutter speeds it may force you to use. Getting an f/2.8 (or smaller f-number) lens gets you 4x or more light than an f/5.6 lens, if used wide open. However, faster lenses are more expensive (although primes can cut that cost down), and larger aperture settings make focus harder (thinner depth of field makes a much smaller target you have to hit), and the wider open a lens goes, the more flaws you can see in image quality (softness, chromatic aberration, vignetting, etc.)

• You can add a flash. You need more light. Add more light. There are many ways to use a flash so that it doesn't give you that deer-in-the-headlights white look most non-flash shooters associate with flash. Bouncing is one go-to-technique. Going off-camera-flash is another.

• You can fake a higher ISO setting. But I wouldn't recommend doing this, unless your camera maxes out around 1600 or 3200. It adds a crap ton of noise. But shoot RAW, and underexpose, then in post, push the exposure back up (i.e., push processing). This is what the "extended" ISO settings in most cameras do.

• Use the blur creatively. Blur indicates movement. Sometimes that can be a good thing. Just make sure that the blur you're getting is from the subject movement, not from camera movement from handholding. The basic rule of thumb with camera shake blur is that your shutter speed needs to be at least 1/focal_length. Some folks also throw in crop factor. So, say, if you're using a 55-200 lens @200mm on a crop body, that would mean using a shutter speed of 1/300s or faster just to eliminate camera shake blur. If you've got good handholding technique.

See also: Why are my photos not crisp?