Possibly not, but does one have to go to church to believe in God?
More to the point, I think it's the fact that the nation's rules were built up from the base of the Christian teachings, and at their basest, the ten commandments.
Our outlook and philisophy is based on Christianity as opposed to Hindu, Muslim, Jedi or whatever else is out there to follow.
Surely anyone wishing to join the armed forces must be prepared to accept all (legal) orders that are asked of them. If not, then they leave - as we are seeing more and more now.
The fact that more and more of the forces are leaving becase they are no longer prepared to tolerate the latest orders and changes to our working pracitces is perhaps what those at the top require.