He didn’t, he decriminalized it and restored confiscated property. And I thought that, at the time he did so, Christianity was pretty widespread. Rodney Stark’s “The Rise of Christianity” estimates there were about 6 million Christians in 300. (He also argues that Christianity wasn’t largely a religion of the poor and slaves, but instead, of Hellenized Jews and of the urban elite.)
I’ve heard that the big, monotheistic religions originated in the desert because the desert is a weird, intense place that lends itself both to hardship and to deep thought. On the other hand, religions like Buddhism come from lush areas and are generally less dogmatic as a result.
Sounds like an oversimplification to me, in a way. But it also sounds like it might be on track, too.
One theory is that the desert setting, with it’s wide open spaces and giant skies, led to transcendant theologies of a grand, external God because the eyes are drawn up and out. India, the source of Hinduism and Buddhism, is a tropical, jungle setting which caused people to turn more inward and introspective, resulting in highly refined meditative techniques and insights and a more internal kind of religion.