Most of what survives of Roman religion is official: temple dedications, state cult, inscriptions paid for by people who wanted to be seen paying for them. Bath is the great exception, because about a hundred and thirty curse tablets were thrown into the spring and stayed there.
The tablets are sheets of lead and pewter, hammered thin, scratched with a stylus, often rolled or folded, and dropped into the hot spring so that they passed to Sulis directly. They are not literary. They are complaints.
The commonest subject by far is theft, and specifically theft at the baths, where people had to leave their clothes. One asks the goddess to act against whoever stole a bathing tunic; another concerns a stolen pair of gloves; others name cloaks, a ploughshare, money, a bracelet. The writer typically hands the stolen item over to the goddess, it is now her property, so the thief has robbed her, and asks her to act until it is returned. The requested penalties are specific and unpleasant: that the thief lose his mind, or his blood, or the ability to sleep, or that he waste away.
Several list suspects by name, sometimes a dozen at once, which is a small social document each time: this is who had access, and this is who I think it was.
Why they matter more than the temple. They are written by ordinary people in ordinary Latin: some barely literate, some with British-influenced spelling, one in what appears to be British Celtic written in Latin script, which would be among the only written traces of the language. Together they constitute the largest body of everyday written Latin from Roman Britain, and they show what people actually wanted from a god: not cosmic order, but the return of a stolen cloak and something bad happening to whoever took it. They were added to UNESCO's Memory of the World register in 2014.
Sulis Minerva herself is worth noting as a genuine fusion rather than a takeover. The Romans did not replace the British goddess; they identified her with Minerva and kept the British name first in the compound. The temple pediment carries a head that everyone calls a gorgon and that is male, moustached and glaring, with snakes worked into the hair and beard, nothing like the Greek convention, and generally read as a Celtic deity rendered by a sculptor working in a classical idiom. The spring gave the Romans a hot mineral source of a kind they valued highly, and it gave the British a goddess who was already there.