tried Google for you but no luck

may I suggest you look up one in Beijing as there are lots of info on them with phone numbers and they may be able to put you on to one in Xian

I know it's the long way round but it might be the best way in the long run

Good luck to you

http://ditu.google.cn/maps?oe=utf-8&rls=org.mozilla:en-US:official&clien...

http://www.xamama.net/thread-448688-1-1.html

The second link has pictures of last year's ceremony at 天教堂 (Tian JiaoTang)陕西省西安市莲湖区五星街, which looks like a pretty big deal, and a nice church.

There is supposed to be a Christian Church or temple as called here in Xian by the name of

Tu Mem Christian Church, unfortunately nI could not find the address, Maybe a taxi driver could take you there if he knew.
Here is a photo of the inside.

http://www.travelblog.org/Photos/992460