Blasphemy was still illegal in many countries, and could even be a capital crime. And even in the ones that weren't, there were plenty of bloodthirsty religious fanatics.

It is certainly easier to argue for an atheist position in an age of science, but atheism itself goes at least as far back as Diagoras of Melos in Ancient Greece. He threw an idol of a god into a fire and said that if gods existed, they would stop the idol from burning. He got chased out of Melos for his trouble. That sort of thing is why, even thousands of years later, if you were an atheist, you didn't admit it.

You could go as far as deism in the Enlightenment, but the Enlightenment was already in full swing in Britain when Thomas Aikenhead was executed for blasphemy due to being an atheist.