The idea that an unholy creature may be able to enter a holy place implies that place wasn't holy to begin with. Or a modicum of other things such as but not limited to: the unholy creature in question is strong enough to resist such powers, God never existed in the first place and it's only the demons we ourselves create that are real, or that God was so disappointed by this church that he just abandoned that place and left everyone to die. Which isn't unlike him.