Worship Me by Craig Stewart

Something is listening to the prayers of St. Paul's United Church, but it's not the god they asked for; it's something much, much older.
A quiet Sunday service turns into a living hell when this ancient entity descends upon the house of worship and claims the congregation for its own. The terrified churchgoers must now prove their loyalty to their new god by giving it one of their children or in two days time it will return and destroy them all.
As fear rips the congregation apart, it becomes clear that if they're to survive this untold horror, the faithful must become the faithless and enter into a battle against God itself. But as time runs out, they discover that true monsters come not from heaven or hell.
...they come from within.