What sort of changes would we have seen through history if Constantine had not converted to Christianity and made it the state religion of the Roman Empire in 380 AD with the Edict of Thessalonica?

This is probably not the right subreddit for this type of question. Historians try to understand and explain what actually happened in the past using historical sources. In most professional history writing "what if" scenarios are pretty rare and usually only limited to a short term scenario. Your question asks some sweeping questions that cannot really be answered historically, we don't have primary sources for the alternate scenario.

You might try posting at /r/HistoryWhatIf or /r/althistory. They are more focused on this type of question.