That's a lovely saying, but I know it isn't true. I know a number of guys who were atheists when the volunteered, atheists under fire, atheists when they went back home, and are still atheists today. Just because folks didn't profess it to you in the military doesn't mean they aren't there -- there isn't much to say about the non-belief in something.
And as a matter of practical statistics, a very significant percentage of the population in Tillman's age range does not have any particular belief in the existence of God and it does not factor into their life at all -- they are non-religious. Therefore, it is easily probable that Tillman was in fact an atheist of some sort. Or at least, I don't find it remarkable.