I find that you can extend if you don't let them bomb you with missionaries and prophets but you lose it fairly quickly anyway as it doesn't generate any pressure, and just having your other cities with other religions generates enough pressure for it to implode, let alone any trade routes or outside pressure from neighbouring cities.I know but maybe when someone first convert your cities you keep your pantheon for some time
However, delaying the conversion was enough to get to the point where whatever benefits I got from it were not that much in comparison to say allying to a city state (and it was feasible to do so money wise) and so didn't much matter at that point if I got converted or not.