Why would they ask if you attend church?
I can see that any organisation that is run by and for christians etc could ask that question (and I guess they are legally entitled to ask it, if it is necessary that to work for the company you must believe in the ethos of the company – example Mission Aviaition Fellowship – a christian organisation) other than that, I dont see what the point of the question is, and as a christian i would be affronted if I was asked that and it had no bearing on the job i was going for