ASL Veteran
Chieftain
The Nazis attempt clearly would have worked. They started the whole (just from thetop of my head) mega holocaust-thing in -44. A year later, they were no more. And during that time they killed ca 10 million jews/minorities. If they would have started this whole thing earlier, say 1940, and put their powder there instead of in the Soviet Union, then they would surely have "eradictated" a heckuvalot of minorities in Europe. (yes I do know that stalin was going to invade hitler sooner or later but whatevs)
The Spanish Inquisition was quite efficent as well. It didn't kill all the muslims/jews in Spain, but pushed away almost all to the Ottomans and Moors. That's two different ways of removing religions.
And Islam (and christianity too for that matter, I just lack examples right here and now) wiped out lots of well organized religions on their way. Zoroastranism, manuism and all that jazz. How can you say that the people there just stopped to believe in it? It was simply a widestretched genocide, as that was how stuff worked in ancient times. You and your army comes to a city of non-believers, raze it, kill the people inside and build a new one. It's when the other religion gets worked into society that pushing it out gets hard.
Alas, the point is you can really wipe out anything and anyone if you just want it bad enough.
Clearly there were invasions and attempts at enforced 'conversion'. I believe it was either Charlemagne or someone associated with his family tree who was making a concerted effort to subjegate and Christianize a "heathen" Denmark. The Teutonic Knights are another obvious example. Those are external wars though (external to the various states involved) which I was not factoring in because the motivation for those wars could either be religious in nature, or there may be other motivations and the religious factor is merely a cover story. You can already happily invade other civilizations and raze their cities so you can already perform those types of activities in Civ.
However, I would also like to point out that at the time Christianity was starting to really take root inside the Roman empire, the empire was being overrun by various ... "Barbarian" for lack of a better term .... tribes. Vandals, Goths, Visigoths, Franks, Saxons, Huns, etc. Certainly you don't think that the process by which these various tribes were "Christianized" was by the sword? The story of how Russia became orthodox christian seems pretty typical of how these 'conversions' take place ... warlord / chieftain is either awed by some form of religious display / victory in battle attributed to a christian symbol / a political calculation that being in league with a christian state is in their interests who then tells his people that they are all christians now. Naturally there will be some resistance to the 'new' god, but when the national leader is telling you that's the way to go, people would listen. The interesting part about the Teutonic Knights is that their opponent (I believe it was the Lithuanians with some Polish assistance - I'm sure someone will correct me if I'm wrong) eventually did convert to christianity yet the Teutonic Knights continued to attack.
In Norway (and forgive me but I'm going on memory here - always dangerous on a international message board) I seem to recall that many Vikings converted to Christianity when missionaries set up tests to prove that the christian gods were stronger than the pagan gods ... something along the lines of a Berserker must pass through a pagan god fire and a christian god fire and the berserker could pass through the pagan fire but not the christian one proving the potency of the christian god. The story of how England's christianity survived the Saxon invasion is similar in that various warlords became christian and their christian relics ... crosses or whatever ... gave them victory in battle proving the old gods as weaker than the christian god. So, my statement that people simply stopped believing in the 'old gods' is accurate. It's probably amazing that Christianity in the western world survived at all to some degree.
After writing all this, I hope I don't come across as a bible thumping christian since I haven't attended a day of church in my entire life. I would also not take the Nazi example of attempted Jewish eradication (within Germany and their occupied territories) as an example of what a normal or average state would be capable of. The levels of organization, logistics, and mechanization of their deeds could not possibly have been matched by any nation prior to the invention of the railroad at a minimum and probably even after that since the entire state would have to be geared and designed with that intent in mind along with having all the secret police organizations and the spare manpower to send house to house to round everyone up. I fail to see how the reconquista could be anywhere near as efficient as the third reich as far as eradicating muslims in spain. Interestingly enough, El Cid (sorry in advance to our Spanish posters but I'm going on memory here) fought both against muslims and for the muslims in Spain and it seems like the two communities coexisted for many many years. I simply can't agree to your last statement which I interpret as "where there's a will there's a way" and will just agree to disagree with you there. Someone can say they believe differently than how they used to when threatened with violence, but will they believe differently in their hearts and minds? Perhaps not.