I think that Constantine the Great was just the last drop in the already filled vase. Changing his decision would have little or no effect on the spreading of Christianity in the Roman Empire.

First of all, Roman religion was intrinsically open to external influences: there are several accounts of foreign gods which were, more or less, accepted or tolerated in the Roman Pantheon or religious behavior. When Christianity started to spread, it was just one among the others.

Moreover, around the 3rd century v.e. the old corpus of the Roman religion, with its values and traditions, was already showing crisis signs: intestine wars were common in the empire, barbarians were already pressing the empire borders and the economic crisis was hitting the empire.

No wonder that there was lack of trust in the old gods, which were showing more and more how they were no longer able to ensure prosperity and safety to the empire worshiping them.

If you really want to prevent or reduce the spreading of Christianity in your world, I think you have to work on those factors I have listed before:

• openness of the religion: make the Roman religion much more closed and afraid of accepting foreign cults
• intestine wars: reduce them, make the central government strong and respected
• barbarians at the border: again, make the central government strong, so that the army is efficient and used for protecting the borders, not for supporting generals fighting for their seat in Rome
• economic crisis: I have no knowledge here to hint at what could be done, if I had I was working in ECB right now.

All in all, I think you are more or less trying to avoid that fruit flies gather around a ripe peach.

I think that Constantine the Great was just the last drop in the already filled vase. Changing his decision would have little or no effect on the spreading of Christianity in the Roman Empire.

First of all, Roman religion was intrinsically open to external influences: there are several accounts of foreign gods which were, more or less, accepted or tolerated in the Roman Pantheon or religious behavior. When Christianity started to spread, it was just one among the others.

Moreover, around the 3rd century A.D. the old corpus of the Roman religion, with its values and traditions, was already showing crisis signs: internecine wars were common in the empire, barbarians were already pressing the empire's borders and the economic crisis was hitting the empire.

No wonder that there was lack of trust in the old gods, which were showing more and more how they were no longer able to ensure prosperity and safety to the empire worshiping them.

If you really want to prevent or reduce the spread of Christianity in your world, I think you have to work on those factors I have listed before:

• openness of the religion: make the Roman religion much more closed and afraid of accepting foreign cults
• internecine wars: reduce them, make the central government strong and respected
• barbarians at the border: again, make the central government strong, so that the army is efficient and used for protecting the borders, not for supporting generals fighting for their seat in Rome
• economic crisis: I have no knowledge here to hint at what could be done, if I had I was working in ECB right now.

All in all, I think you are more or less trying to avoid that fruit flies gather around a ripe peach.