Ahh... the ol' Ayu-religion debate ...
As said many times before this, the fact that Ayu wears a cross or sing about "Maria" doesn't mean she's Christian, just as surreal said with the Yin-Yang stuff. It's all just symbolism for the PV and nothing more, so unless Ayu one day announced she was Christian or was seen going to a Christian chruch, I don't think she is, but then again, ... without prove, there is no definate truth yet...