They are different though. If you only look at your own cities then sure, they end up being functionally the same since neither can receive foreign religious pressure. But Spain also denies foreign religious pressure to CS allies, still generates religious pressure of its own to foreign cities, and instantly converts cities on capture/settle. In comparison, Spain is much stronger at spreading its religion while the Celts get a stronger religion from their pantheon but has worse ability to spread religion. Or in other words, you can't just say they have the same religious mechanics just because neither of them can be passively converted. India also functionally cannot be passively converted, but at the end of the day all 3 of these civs have significantly different religious mechanics.