The religion stretched across many galaxies, and included monks in its order. C'rizz's father Guidance was a senior leader, advising the Church's titular leader Daqar Keep. (AUDIO: Faith Stealer, The Next Life)
In the Divergent Universe, the natives detected a small blue planet working its way through the galaxies, exterminating all life in its path. The Eutermesans watched as galactic empires retaliated against it to no effect, and came to believe that the planet was part of the natural order of things, and it should be supported. Their central creed was "All things must die."
The Church believed that life itself was founded on that small blue planet, and all life must eventually return there. However, they were also greatly willing to help things along by killing people themselves. To this end, their acolytes were well trained in the arts of death.
C'rizz lived in a peaceful village under the tutelage of Guidance, but with his planned marriage to L'da, C'rizz was planning to leave the Church. A brutal attack by the Kromon left the village in ruins, C'rizz a slave, and L'da a Kromon brood queen. C'rizz killed L'da under the tenets of his religion, and left to travel with the Eighth Doctor.
Eventually the Doctor and C'rizz arrived on the small blue planet at the same time as Daqar Keep and Guidance, where it was revealed they were all being manipulated by Rassilon as part of a plan to escape the Divergent Universe. The Church's role was to bring the key that would open the gate between universes. Though the meeting between Keep and Rassilon had happened repeatedly over many cycles of the universe, the new influences of the Doctor and Zagreus allowed the gate to open, and in the end the Doctor (and his companions) escaped whilst everyone else was left to repeat the Universal cycle. (AUDIO: The Next Life)