Here is the full list of all 39 Cathedral achievements.
Destroy an electric orb in Necromancer's den using the lantern
Kill Ardur
Find all ammo upgrades for the Magnus Apparatus
Find all armor augmentations
Find all map pieces
Find all ammo upgrades for the Flintlock
Destroy the Conduit
Die one hundred times
Find and activate all portals
Acquire all health bottles
Return 100 books
Kill 150 skulls (of any kind)
Have more than 25 000 gold stored in the bank
Kill Oloc
Kill the blood knot
Return 80 books
Kill Nidlong
Find all hearts
Find all charms
Acquire all Ouroboros amulets
Slay all gargoyles
Buy the Onyx armor
Kill Thalia
Return 60 books
Return 40 books
Buy the Tourmaline armor
Kill Salemacke
Kill 100 skulls (of any kind)
Have more than 10 000 gold stored in the bank
Kill 150 blobs (of any kind)
Kill the Necromancer
Return 20 books
Kill 50 skulls (of any kind)
Buy the Emerald armor
Kill 100 blobs (of any kind)
Kill Augr
Kill 50 blobs (of any kind)
Have more than 50 000 gold stored in the bank
Complete the game without upgrading the armor