Of course, some would say living under Roman Empire style repression makes for a better quality of Christain. Maybe they have a point.
Yes, this is an excellent question. Is a society based on enlightenment principles good for Christianity because it allows religion the freedom of separation from the state? This has been the reigning philosophy of the last few decades, based on John Courtney Murray and others.
However, a more sophisticated analysis might point out that the "opposition" of enlightenment society is much more insidious than the opposition of crude repression, working as a continuous corrosive and effecting a much more complete destruction of faith in the ultimate analysis. It's like the difference between the Leninist approach and the Gramsci approach. Gramsci is much more dangerous.
Would Christianity be better under communism?
The comparison makes it seem that communism is not a branch of enlightenment liberalism. Certainly it is true that they have been seen as separate, and communism has defined itself in opposition to "bourgeois" society. But when one examines the foundational principles, should they be classified in the same genus or a separate one?
Good question. It might be argued that Christianity thrived best, at least in America, during the period of time referred to by some as the Old Republic. That time, and that system, before FDR's modern welfare state was created. Before the State replaced the Church as the ultimate authority and moral arbitrator, and Man replaced God as Supreme Being.