The term "atheist" was applied to Christians in the Roman Empire, not because they didn't believe in any God, but because they objected to others' belief in gods that didn't coincide with their conception of divinity. They were opposed to one or more gods. In the modern era, where monotheism has become dominant in many parts of the world, the meaning has evolved, but the bit in "History of the World" where Mel Brooks made fun of the Jews only being able to afford one god was kind of accurate. For the most part, Romans were okay with whatever tribal or local gods people prayed too, as long as they also publicly supported the standard pantheon, by which they proved their respect for the culture and their participation in the Empire. Those who refused (Christians and Jews, primarily) were seen as subversive for not falling in line with the dominant tradition, similar to the way some people are currently painting Muslims in many parts of the world.