Tebryn isnt actually religious at all. But when we talk civ sometimes we muddy the concept of a civilizations state religion and the leaders religion. Tebryn wasn't above allowing religions to exist among his followers as long as it served his purposes. The Emrys was a fairly popular religions and acted as a missionary religion in other civilizations to find and recruit those with magical apptitude and send them off to the Sheaim lands to serve.
Although there wasnt a state religion per se in the D&D game the worship of Bhall, Agares and Ceridwen were popular and Tebryn wouldn't have been above declaring one of them a state religion if it suited his goals.
As for the other, its important to note that thier name is the Stewards of Inequity, not the Stewards of Inequailty. They don't care directly about slavery or any human rights conditions that inequality would imply. The only thing they struggle to maintain is a dissproportionate allocation of the weath (and therefor power) within whatever culture they are acting. Think of them as the Masons are sometimes attributed during medieval times, full of secret symbols, a clandestine gathering of powerful men working behind the scenes to make sure they maintain their power.
In some ways they are a lot like the Council of Esus except the Stewards of Inequity are almost entirely focused on the business aspects of operations, and have very little interest in anything outside of that. Its also worth noting that many Stewards don't directly worship the god of the religion (mammon). But they do follow the rituals of the Stewards (which pay homage to mammon, even if not all the members realize it) and mammon actually prefers that they follow him by pursuing their own greed.