The Romans were largely indifferent to their citizen's various beliefs. It was the rise of monotheism that led to the concept of orthodoxy/heresy, and the consequent persecution. That might not have happened in a consistently pantheistic history.
Of course, I agree with you that it's a all...
...and therefore might have quashed Licinius's equally pro-Christian stance. Would this have meant monotheism withered, a relatively benign pantheism prevailed, and the development of a reason might have flourished in less adversarial intellectual circumstances than actually prevailed?
If Constantine has not declared himself (and the Empire) Christian, could we have fast-tracked to an "Enlightenment" of some form, without the need to for the intervening Dark age / Medieval age?