I must confess the whole "we don't celebrate Hallowe'en because we're Christians" thing rather shocked me when I first arrived in the UK. I'm from the US, where EVERYONE celebrates Hallowe'en and gets into the spirit, regardless of their religion. There are few people who can get more po-faced about their own religious observance than Americans, but this holiday has become, like Thanksgiving, completely divorced from its religious origins and therefore everyone can just have a good time.
On the other hand, I had always viewed the UK as intensely relaxed about religious observance (and I mean that approvingly!), so to see people in this country decline participation to the point of being offended just boggles the mind.
Sorry OP, I'm not sure I have any real advice about your situation - you are definitely not being unreasonable in having the decorations on your door, but as some wise posters have noted, you have to live with this person day in and day out. While she needs to acquire a thicker, more offence-proof skin, making it your job to give it to her could in the end make you more miserable than you would be if you just let it go and kept your personal decorating ideas confined to your room.
If you can't ask her calmly and rationally why the decorations offend her (and I can see why you couldn't/wouldn't), and be prepared to compromise on hearing why, then unless you want lots more door slamming you need to just get them out of her sight.
And YANBU for putting them up on the 18th.