Not true. The switch to enable Muslims is in the game right now. It's just like having an .ini file with various game settings that you don't want to build a GUI specifically for. You may not want to provide technical support for people who enable that function (which is fair enough) but you can't pretend that it isn't a feature of the game that everyone has *right now*.
When you are doing is developing the precedent that you're are willing to take away existing content in order to sell it back to the customer in the form of DLC, which, from a consumer's prospective, reduces the value of any base game because they can expect that features will be actively *removed* for reasons besides them being "broken" or causing technical glitches.
As I see it, you had three ways to handle this properly.
1. Never release the ability to play as other factions, so you would not be taking away existing content in order to sell it a second time to the player. (The cat's out of the bag on this, and you can't do that now)
2. Release a full-fledged expansion for all of the other religions so that you can continue supporting one line of code, while still not removing the ability to enable the other religions for play, even though they have somewhat less content than Christians. (Could still do this, but you'd need to expand this DLC into a full-fledged expansion which covers a great deal more, and it's probably too soon for an expansion for the game.)
3. Treat content expansions for various religions like "mods" where they merely plug in to enable additional content, without affecting the ability to play as any religion if the player ticks that flag. This creates the issue of needing to support multiple code lines however, and may be more prone to piracy given how you don't have a special notch for your dlc to plug in as a mod using encryption keys only you have access to.
The way you are handling it, however, is arguably deceptive as a business practice and certainly harmful to the value of your product.
Edit: Jonan posted after I wrote this. His response alleviates my issues. I would like to remind, Gars, however, that just because a function is in a text file doesn't mean it's not game content, and you shouldn't pretend as such.