the ancient Roman world, where all religions were tolerated as long as one sacrificed something edible to the Roman gods in their temples, thus feeding the priesthood and their slaves, sort of an additional tax everybody had to pay. Jews and the new Jesus sect refused to do this, which is one reason they ran afoul of the law. Everybody else aligned with the equivalent of their own gods and goddesses. Cults of Mithras (Middle East) and Isis (Egypt) were as popular as the home grown gods.

Likely most people would follow something, we're a superstitious species, so instead of waving our arms in the air at a megachurch we'd be dancing naked in a forest or sitting zazen or coming together to knock back some synthetic ayahuasca. People will be secular in their daily lives in order to hold down their jobs but people who are atheists will still be few.

Some of the stuff will be dangerous and some fairly benign. I just hope the heavy handed misogyny of Christianity is what disappears first. A two thousand year curse is long enough, thanks very much.