THM worship the twin gods of Nald'Thal.
http://ffxiv.gamerescape.com/wiki/Nald%27thal
Nald'thal is the overseer of transactions and the underworld, and god of commerce. He commands the element of Fire and is associated with the tenth moon of the Eorzean calendar. Nald'thal is the single manifestation of the deific twins Nald and Thal. He is most often depicted as a discerning merchant holding a balance.
Nald'thal the Traders is one of the Twelve. He is the patron deity of Ul'dah where two great halls, devoted to his two aspects, can be found in the eastern and western sections of the city.
Nald rules over the world of the living, and Thal keeps the realm of the dead.
The Thaumaturge, BASE CLASS, are the priests of DEATH. They are based in the Arrzaneth Ossuary, their temple of death and worship of Thal, in Ul'dal.
The Milvaneth Sacrarium is on the other side of Ul'dah. This is the temple of life and the worship of Nald.
The THM lore is death and spilling of blood, but that is only half of it. There is a whole other half of the faith devoted to LIFE. This whole other half of the faith would easily fill why White Mages are THM and also why THM BASE CLASS is the polar opposite to white mage. IT IS A DUAL SIDED FAITH.