Symbolism, Imagery, Allegory
Finally, some answers about the afterlife.
Oh, wait.
The afterlife in Stranger in a Strange Land is kept ambiguous for a reason. Foster and Digby go to an afterlife that seems in line with the traditional picture of Heaven: angels with wings and halos, looking down on Earth. Stuff like that.
But at one point, Foster asks Digby if he wants to take a day off at "the Muslim Paradise" (25.10). Here, Foster suggests that the afterlife in Heinlein's universe is a combination of all the afterlives imagined by humanity, and, perhaps, those created by extraterrestrial imaginations, too.
When Mike dies and goes to the afterlife, he gets to continue working to change the world. These aspects of the afterlife suggest a two-way cause-and-effect relationship between the afterlife and life-life. What we learn about in the world affects our view of the afterlife, and this view of the afterlife changes the way the world is seen by those living in it.
Check out the "Setting" section for a discussion of more earthly settings.