GP does not make a fair point. We're specifically talking about classical antiquity which was a fairly bounded world. Warrior god cults, like that of Mithras, didn't have a strong role in the overall state and direction of the empire. They weren't major players and it is actually perfectly fine for terminology and understanding to focus on those.
Christianity is the prism through which the Romans later viewed things and through which the heirs of classical antiquity did. This isn't parochial, this reflects your general dislike of Christianity's dominance. But I don't actually have to make a normative argument that it should be, just the positive point that it is.
"Pagan" is a widely-accepted way to refer to Rome's old polytheistic religious traditions, which existed, but not unchallenged, around the first century.