The title Stranger in a Strange Land is a straight-up shout-out to the Bible. In Exodus 2:22, Moses names his son Gershom, for "he said, I have been a stranger in a strange land." Wait, what does Moses naming his son have to do with a Martian trying to grasp human culture in the future?
Not much on the surface, but the connections between Mike's and Moses' lives are actually pretty numerous. Driven from their homelands at a young age? Check and check. Given instructions to return to their homeland later in life? Check and check. And once there, attempt to liberate their people from social bonds? Major check and check.
Moses frees the Jews from being slaves in Egypt, and Mike frees his water brothers from the social mores of Western culture. Each performs miracles to help in their task. In the end, Moses and Mike try to take their people into a promised land, and although neither lives to see the outcome themselves, they go on to become prophets in their own right.
The title also connects Heinlein's novel with the Bible in general. Many other prophets in the Bible, such as Jesus, can be seen to have parallels with Mike's story. Like Jesus, Mike challenged what his culture considered normal behavior and thought. Each tried to show their people a new way to consider life, love, and what it means to be human. And many of these prophets also suffered terribly as a result—in this case, both Mike and Jesus are martyred. (And don't forget the title of Part 1: "His Maculate Conception"—play on words, anyone?)
Granted, Heinlein is not retelling the story of the Bible in a science fiction setting, but these connections can still be important to your understanding of the novel. Whether you want to focus on Moses or Jesus or both, each parallel drawn from the title can help us open up the novel in new ways. Perhaps we are supposed to see the story as myth; maybe we are supposed to reconsider the roles prophets play in society; or maybe, just maybe, we are being welcomed to become strangers in our own land. The possibilities are endless.