The Gods Of Mars (1913) by Edgar Rice Burroughs

   Edgar Rice Burroughs was not a fan of organized religion. He recognized it for what it all too often is: a tool by those in power to manipulate others. He knew that those in charge were just men, men with no more knowledge of the divine than he had himself. False religions were common in his fiction, but never more so than in The Gods of Mars.
   It was only a matter of time before John Carter returned to Mars. A Princess of Mars was a great success and Burroughs' readers demanded more. Fortunately Burroughs had a lot more to say about Carter and the red planet. Unlike some of his other series, the Martian tales never declined in quality and The Gods of Mars is the best of the series. Here John Carter faced his greatest challenges and faced  the deadliest dangers of his career. By this point Burroughs had mastered his profession and with Gods he delivered one of his most inventive and action-packed novels.
   The story begins with John Carter stranded on Earth in 1886. For ten years he had longed to return to his beloved Barsoom. Now, standing before the Hudson River, he again felt the pull of the red planet. Just as before he experienced a moment of cold and darkness as his soul is pulled mysteriously to Mars. He wakes to find himself in the very last place he would want to be on Barsoom: the Valley Dor.
   The Martians had a very peculiar religion. Eventually, if they managed to live long enough, every Martian took a voluntary pilgrimage along the River Iss, to the lost Sea of Korus in the Valley Dor. The Martians believed that the Valley Dor was heaven, a paradise where they would be reunited with loved ones and enjoy everlasting peace. John Carter discovered the ghastly reality. The Valley Dor was a deathtrap where pilgrims were torn apart and devoured by weird plant-men and the great white apes of Barsoom. Any survivors were enslaved by the Holy Therns, priests who also just happened to be cannibals.
   Here Carter encounters a group of pilgrims who are being attacked by the plant men. Among them is the green Martian Tars Tarkas, John Carter's best friend. Carter and Tars Tarkas survive and together they fight their way through the valley and into the domain of the Therns. They encounter the mysterious beauty Thuvia, who becomes a very important character in the series, and together they plan to escape and expose the false religion for the terrible lie that it is. The problem is that to return from the Valley Dor is heresy in itself, one punishable by death anywhere on the planet. John Carter not only has to fight men and monsters, he also has to try and defeat a deeply-rooted cultural superstition. A hard task, even for the superhuman John Carter. Once he manages to escape the Valley Dor his troubles are only beginning.
   Along the way Burroughs invents some of his most interesting Martian cultures yet. The Holy Therns are both menacing and repulsive. The Therns' enemies are the Black Pirates of Barsoom. The Black Pirates inhabit  a subterranean world by an underground sea and are particularly fascinating.
   Burroughs wrote many extremely entertaining books, but The Gods of Mars is something more than that. While it succeeds brilliantly as escapist adventure, it is also a story about the crippling power of cultural superstition. Fighting monsters is one thing. Fighting a whole world's ancient religion is something else again. It is here, in this book, that Burroughs takes John Carter to his greatest level as a hero.
   This is Burroughs at his best. Well worth checking out.