Thayans are known for a relative irreverence toward the gods; whereas much of Faerun is devout, with many who put their gods first, for most Thayans, the gods are definitely further down the list of priorities. That's not to say that they are stupid or suicidal. Since the gods are very real, very powerful, and very harsh to atheists, Thayans have patron deities like most other inhabitants of Toril. They are more likely to weigh the costs and benefits of patronage in a calculating fashion, however, choosing gods that fall in line with their plans and offer interesting afterlives. They raise churches and make donations, but as much for personal glory as glorification of the gods. Services are attended sporadically, and prayers are largely made up of lip service and litanies personal goals and vendettas. Holy days offer opportunities to display wealth and power, so why not celebrate? The general attitude in Thay is this: The gods are bigger than you, so don't tweak their noses.
A strange kind of religious freedom reigns in Thay, not because individual choices are respected but because Thayans know their own history. The ancient Imaskari empire fell because they kept slaves from communing with their gods, and the Thayans are not about to make the same kind of mistake. Thus, it is against the law to magically or influence anyone to change alignment or otherwise force them to choose particular patron deities, even if powerful masters and parents don't approve. While no worship is suppressed outright, some faiths are openly ridiculed (namely the Mulhorandi pantheon, Mystra, and Velsharoon). Although she is the current goddess of magic, Thayans tend toworship Azuth, if they must choose a "god of magic" at all; at least he's wielded magic for a long time. With the magical weather net over most of the country, Thayans disdain to follow Talos, and he hates them in return.
Slaves are not forbidden the worship of their deities but their practices are regulated. Slaves cannot raise temples or gather in groups larger than 10, but they are allowed to make shrines and religous goods, like holy symbols, provided they acquire the materials legally. Any shrines must be in the slave quarter or another designated area out of sight of main thoroughfares. All prayer in Thay is legally deemed to be private, even with slaves, and spying on prayers is a serious offense. Slaves are not usually given time off for individual holy days but once their chores are done, they can choose to spend time in worship. The major seasonal festivals in Faerun (Greengrass, Midsummer, Highharvestide, the Feast of the Moon, and Midwinter) are granted as rest days for all slaves, despite the grumblings this causes in the upper classes.
It must be noted that Thay is a place of religious turmoil for slaves, since their harsh treatment can cause them to reconsider the gods they serve. While some slaves experience a renewed faith, many find their gods' promises to be hollow as their servitude drags on. What does Chauntea's bounty matter when the masters won't allow any more than the barest ration? Light and love wither for the thousands of slaves doing the hardest, dirtiest labor in Thay, far away from any hope of rescue. Some turn to deities of bitterness like Beshaba and Shar; others turn to Loviatar and torment other slaves. A few follow Kossuth in the hopes of gaining favor with the upper classes, with whom he is incredibly popular. Since Malar's church offers the High Hunt, one of few ways for slaves to earn their freedom within Thay's borders, he is more popular wherever Hunts are held.
Humanoids are multitudinous in Thay and often serve as guards, slave patrols, and laborers. They are allowed to pray anywhere but must not shirk their duties. They can raise no temples aboveground, and any shrines must be well outside of Thayan settlements. The lizardfolk of the swamps in Surthay must keep their faith within their damp borders but can raise temples there, as per the agreement with the tribe. Giants and other denizens of the mountains keep their gods to themselves and try to have minimal contact with the humans. Any nonhuman group can be punished (or simply wiped out) if their religious rites seriously damage Thayan property or interests, so they tread carefully.
Two cities are notable for their religious significance in Thay: Bezantur is called the City of a Thousand temples because it has the greatest religious diversity of any place in country (and perhaps all of Toril). It caters to citizens, slaves, and regular visitors to the port, so it is one of the only places one can find proper churches to good deities. Temples to evil deities are allowed out in the open but are built in their own quarter to avoid scaring foreigners, with temples to neutral gods serving as a buffer zone. Eltabbar is known as the religious heart of Thay, with some of the grandest temples anywhere, but all official churches are constructed on an island known locally as "the zoo." Eltabbar is also the closest major city to the symbol of Kossuth's worship: the volcanoes of the Thaymount.
While there are exceptions to every rule and good Thayans do exist, the vast majority of the country's gods are evil, lawful, and neutral. Slave gods are overwhelmingly good-aligned (which is part of why most citizens mock them). Foreigners tend to believe that because of their attitudes, Thayans are atheists or only worship a few deities, but there is great diversity in practices and a stocked religious calendar. Major holidays are the only times that some Thayans go to church, and great displays of power and wealth are not common on these days. Slave uprisings sometimes happen on or near holy days, as well.
You can get a peek at the overview of Thay's gods and holidays below, or you can download a copy for yourself at the link above.
Resources are free for personal use; please do not offer them for sale or claim them as your own work.
Please do not repost material elsewhere; link to this site instead. Thank you, and happy gaming!