In Stranger and a Strange Land, religion comes in two varieties. The first hews closer to spirituality than religion, but for the sake of ease, let's just keep them under the same umbrella. This form of religion provides people a means of brotherhood, a way to come closer together than they can in other social institutions like government, Facebook, or the office water cooler.
The second religion is religion as an institution or organization. This religious form uses "Rules and Order" to limit the freedom of its members by saying, for example, that only certain types of sexuality are acceptable and only certain foods can be eaten on particular weekdays. On the other hand, Heinlein suggests that America allows organized religion insane levels of legal freedom, which can give members more freedom than they can achieve individually. So religious organizations become a double-edged sword. One religion can grant its members freedom from social norms while another will have a televangelist preaching even more restricting rules to his congregation.
Remember that these two varieties of religion are not mutually exclusive in Stranger. The Fosterites have ceremonies that bring their members closer together in bonds of brotherhood, but they also enforce rules that limit their members' freedom (like telling them to buy alcohol only from church-approved outlets). Is this art imitating life? What do you think?
Mike does not take issue with religion or spirituality as a belief system, but with using religion to create an institution that rules over people's liberty.
Despite arguing against many well-established religious beliefs, there are no true atheists in Stranger in a Strange Land.