This is a good thread to answer your question, but to sum up:
Mainly because until the events of Diablo 3, demons had difficulty entering Sanctuary, and their manifestations were not capable of drawing upon their full power, and because the protagonist of each game gains experience and equipment over the course of their story (so an even challenge at the ...
Here is some of the Plot from the Diablo Wiki describing the cathedral and monastery.
The setting of Diablo includes the world of Sanctuary, as well as
Heaven and Hell. After eons of war between angels and demons, the
ascension of man prompted the three Lords of Hell (including Diablo
himself) to seek victory through influence, prompting their exile ...
There are two known major religions in the Diablo world (from the D3 lore):
The Temple of the Triune; in reality Diablo, Mephisto & Baal in disguise
The Cathedral of Light, set up by the rogue archangel Inarius to counter the Triune.
Some of the orders such as Zakarum are sects or splinter groups following the Cathedral of Light. Since the ...
As far as I can tell, this line by Gheed is the only mention of Melra in the game.
The Sister's Burial Grounds is where the Sisterhood of the Sightless Eye bury their dead. When the player is sent there to stop Blood Raven, Kashya tells them
Now, you'll find her in the Monastery graveyard raising our dead as zombies!
Gheed is wondering if Melra is among ...
Philosophical point, but is it still a religion without a need for belief?
Philosophy aside, the realities portrayed in Diablo include a very real and obvious state of the realms of Heaven and Hell. People may still worship the beings in Heaven (or Hell for that matter) and there is still a need for buildings to facilitate this but there's no real need to ...