The world, as the Greek philosopher Thales wrote, is full of gods, and all deserve respect and honor. Such a generous understanding of the nature of divinity allowed the ancient Greeks and Romans to accept and respect other people’s gods and to admire (rather than despise) other nations for their own notions of piety. If the Greeks were in close contact with a particular nation, they gave the foreign gods names of their own gods: the Egyptian goddess Isis was Demeter, Horus was Apollo, and so on. Thus they incorporated other people’s gods into their pantheon.
What they did not approve of was atheism, by which they meant refusal to believe in the existence of any gods at all. One reason many Athenians resented Socrates was that he claimed a divinity spoke with him privately, but he could not name it. Similarly, when Christians denied the existence of any gods other than their own, the Romans suspected political or seditious motives and persecuted them as enemies of the state.