Here is the full list of all 12 Doorways: Holy Mountains of Flesh achievements.
Find All the Relics
Find All the Notes in the Game
Find All Notes from Celia Torres
Finish The Doorways Saga
Finish Act 3: The Temple
Find All Notes from Juan Torres
Find All Notes from Thomas Foster
Find your First Relic
Open the Gate to The Temple
Finish Act 2: The Mansion
Find All Constellations Notes
Finish Act 1: The School