Get Smart

In Poul Anderson's Technic History, humanity, and therefore also human religions, spreads out into the galaxy: 

Orthochristianity on Dennitza;

a Muslim-Buddhist synthesis on Altai;

a Jewish colony on Dayan.

A brief and readable account of the three Abrahamic faiths and of three contemplative traditions is World Religions: A Dialogue by Ninian Smart. A fictional conversation between adherents of six major faiths or paths, this books reads almost like a dialogue in a novel  except, of course, that these characters have an unrealistic ability to sum up the discussion so far and to avoid digressions and repetitions. It is all very friendly, as it should be. If you want to understand some of Anderson's characters better, then I suggest that you read at least this one book by Smart.

Other main aspects of Anderson's works, of course, are physics and history and other background reading has to be recommended for them. What a well-informed and comprehensive sf author.