Heinlein has explicitly said that Stranger in a Strange Land was not an effort to convince people to live in any particular way.
>“I was not giving answers. I was trying to shake the reader loose from some preconceptions and induce him to think for himself, along new and fresh lines. In consequence, each reader gets something different out of that book because he himself supplies the answers... It is an invitation to think -- not to believe.”
https://archive.nytimes.com/www.nytimes.com/books/97/09/28/l...
Advocating that people should be willing to reconsider their deeply held cultural beliefs was a major theme in Heinlein's later works.
See also: The Moon is a Harsh Mistress
That said, i've only read a few of his more famous novels.
Every zealous over-reaction is typically grounded in some small truth. If it was totally separate from reality it would be much easier to fight; the kernel of truth is what makes extremism dangerous.
For what it's worth, Soviet intelligence did infiltrate large parts of American society during the Cold War [1]. This historical fact is often obscured by accounts of McCarthyism and the Red Scare. While McCarthy was a liar and an alcoholic with little to no knowledge of actual Soviet spy networks, the networks did in fact exist.
[0] https://www.azquotes.com/author/6509-Robert_A_Heinlein/tag/r...
[1] https://en.wikipedia.org/wiki/Soviet_espionage_in_the_United...