If you mean it's ideology, in the sense that Zizek means ideology– a notion that has little to do with how reality functions and serves only for some group or another to gain– then no, Christianity isn't ideology. For one, the most famous Christians all met with horrible martyrdoms, which is not what I'd call a gain from their preaching. Second, Christianity is very clear that suffering is real. This is not something you want to tell people you are trying to sell on the whole idea.