We don't really do "what if" questions on AskHistorians. There are other subreddits for that. I'd reformulate your question as something like, "How powerful was the Christian religion in the early 4th century Roman Empire? How decisive was Constantine's conversion to Christianity's spread?"

Sorry, but your submission has been removed because we don't allow hypothetical questions. If possible, please feel free to rephrase the question so that it does not call for such speculation, and resubmit Otherwise, this sort of thing is better suited for /r/HistoryWhatIf.