Finding information about the religion is the lizardmen is not hard at all - there are lots of clues, influence in present-day religions, archeological sites, and off course: the people. The lizardmen's empire is long gone, but the now-nomadic lizardmen still practice their old religion. According to research, the lizardmen continued the religion of the Ancients, belief in the balance beetwen Order and Chaos: You give, you take. After the Maessepreost Despotanfras race became extint, the lizardmen started to employ men from their own race as priests, who led the rituals. All we know is that the warriors worshipped chaos, so that would give them superhuman-like strenght, while the average townsfolk prayed to the Order, so they would get good harvest. That's what the core of the religion was like and still is like.
It is unknow if they believed in gods, spirits or anything at first, but it is known that later, they started to believe in a number of deities and associate them with either Order or Chaos.
Due to the freedom-loving nature of the Lizardmen, they refused to pray until the life conditions started to grow worse, which the priests exploited. For some reason, as they started to pray, the everything became better.
Later, their religion started to beome much more united. Priests would also become early philosophers, and started to believe that they "can't believe in the vacuum, there must be a person, or more who created the world and still watch us".
So they came up with the idea of worshipping a number of deities.
The later-adopted deities they associated with Chaos were Opochtli (God of War), Camaxtli (God of Hatred, Fear and Tyranny) and Chalmecatl (God of Movement, Speed and Air).
The later-adopted deities they associated with Order were Ometeotl (God of Fertility, Harvest, Food and Family), Oxomoco (Goddess of Astrology and the Calendar) and Atlaua (God of Water, Sea Creatures, Life and Youth).
Unfortenetely we can't say too much about their religion. The priests of the Lizardman Empire used the old Ancient Temples to sacriface animals and goods in order to please the gods. But like many religions, it started to gray as the time passed. Expansion was stopped, people stopped to pray to the Chaos, and the religion become more and more corrupt with the sarifaces. They even started to practive cruel and inhumane man-sacrifaces. The ballance broke and problems rose again. However the Empire was already too big to support itself, and to care about such small problems. Some people heeded the priests, while some - like the Emperor - didn't, so the religion and the imperialist goverment started to weaken each other.