I believe Ayumi is just like the normal japanese religion saying goes:

"Born Christian, live Shinto, die Budhist"

I feel she doesn't take religion THAT seriously, maybe she'll casually go to a shrine, there's always New Years. But she's not some sort of religious fanatic like a ton of Americans.

I've also noticed that crosses and churches are a sort of fad/fashion statement. It's more of an aesthetic thing rather than it's religious counterpart meaning.