Religion is not the same as faith. Belief in a church or in a holy book is not the same as belief in godliness, or higher ideals. Duty is the sense of obligation that transcends self-interest. Loyalty is much the same. Heinlein was certainly an idealist, but just as certainly he would never allow others to try and nail him into their system of ideals, especially when so few of them made much effort to live by their own.