I don't know how many times this has occured, but it has happened often enough to cause me to pay great attention to where I sit at church.
My church is very large with a great deal of dark corners that would be somewhat inconspicuous to someone running through the door.
Hopefully it would give me enough time to get my gun, but these things surely happen very very fast with no warning.
Even if you were to get a shot at the person doing this....it would likely be a very dangerous shot to take because of crowds of people. I agree that we should all KNOW what we would do in this situation before it arrives. I once had a pastor that told me he would say a prayer for the person that did this in his church, but it would be after his magazine was empty

He married my wife and I. Great guy and amazing with his 1911.
Jason