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There are many possible ways of keeping England Catholic. Of course the butterfly effects are hard to completely predict but it is reasonable to assume that eventually Britain would unite in some form as it can be considered a "natural union" ie because of geography. I'm guessing that if England stayed Catholic the UK or whatever it would be called in that timeline would also include Ireland. Now that the main barrier of a united British Isles is removed can anyone think of likely reasons why this would not happen?