The Doors of the Universe

Children of the Star trilogy, Book Three. Noren had found faith in the Prophecy’s fulfillment in the face of overwhelming odds. Yet the more he learned of the grim truth about his people's situation, the less possible it seemed that their world could ever be changed. It would take more drastic steps than anyone imagined to restore their rightful heritage—and he alone could do what was needed.

The three novels in this trilogy were originally published in hardcover by Atheneum as Young Adult fiction, although unlike the first, This Star Shall Abide, this one is rarely of interest to readers below high school age. In 2000 an updated omnibus edition of the entire trilogy was republished as adult science fiction under the title Children of the Star. Ebooks of the three novels were issued under the author's personal imprint Ad Stellae Books in 2008 and a paperback edition of the omnibus in 2012. The covers were changed in 2015 and again in 2015 without any change to the books themselves.
From the reviews:

“Although it is the third book of a trilogy, The Doors of the Universe stands powerfully by itself as a quest for survival on a planet that is basically alien to the Six Worlds’ life forms. This is much more than an adventure story. It is one man’s realization of the need for change and his slow acceptance of the responsibility to lead that change. . . . One never gets bored with the story and it haunts the reader long after it is finished.” —Journal of Reading

“This Star Shall Abide and Beyond the Tomorrow Mountains ... serve as solid foundation for this powerful culminating volume that treats in far greater depth the philosophical/ethical/religious issues raised in the earlier books. . . . Engdahl’s latest story is certain to appeal to the thoughtful good reader.” —Booklist

“Engdahl again proves herself a master storyteller in this third book of her sci-fi trilogy. As a converted sci-fi hater, I am again impressed with the depth of ideas that she explores.... The constant twists and expansions of plot keep the reader’s attention from lagging.” —Provident Book Finder, Scottsdale PA

““Engdahl can make a reader forget her characters are on another planet, forget that they may not be human in precisely the way the people on this planet are, forget the problems Noren is facing are simply fiction. . . . Humanity, she says, transcends the definitions of outward form and physical location.” —Ypsilanti Press
“This book and its companions, This Star Shall Abide and Beyond the Tomorrow Mountains, will become classics of science fiction. They will not, unfortunately, be popular [with teens] because the intellectual level and reading difficulty will restrict their circulation to the more intelligent high school students.” —Children’s Book Review, Brigham Young University

[The novels of the trilogy were first] marketed as young adult books, but there is nothing juvenile about them.... Engdahl tells an important and pertinent story, a parable about the ethical uses of mythology. Under what conditions is it justifiable to keep people in ignorance? Should knowledge ever be sequestered? Who can decide what is good for other people? Is it legitimate to use religion to control a population at risk? Engdahl opens all these questions and more in these novels, which read quickly; the writing is plain and direct, the only lyricism in the stunning liturgical language she invents." —San Jose Mercury News

"You will almost certainly come away enriched from Children of the Star, but ... this is not an easy read, in any sense of that term. Which is perhaps another way of saying that it's an extremely good book." —InfinityPlus