I can only speak to my own observances. Yes, in the military there are a lot of very religious and committed Christians. However, I was never in a command where the climate was one of forced participation in religious services, even in a combat zone. Being a nominal Christian I never felt forced to participate or not participate in religious services by anyone. I remember having an Egyptian Marine who was a muslim (a nominal one at that) who the unit went out of their way to accomodate. This was long before 9/11.
as someone whos been there, i can say that some people in the chain of command do try and force it on you which just pisses everyone off.
almost everyone is a christian, but we dont want it force fed to us. nor do we like mandatory church time either.
it definitely happens. more than you think.