The events in "Endymion" take place about three centuries after the events of Hyperion. As we know, after the Fall (as they called the destruction of the null-portals), all worlds were cut off from one another and, in many of them, chaos reigned.
As always in such cases, a certain force is found that is able to calm everyone down using a power-based method. And such a force was found! It turned out to be the Church in Christ, which during the Hegemony era was almost the most dormant religion among those existing then. The main lever of control over people in the Sacred Order is the cross-form, which grants factual immortality to anyone who wants it. But how often this happens—there are many opponents to both the regime and this immortality! And the church, whose actions strongly resemble the Empire from Star Wars, tries to force everyone to accept Christianity by force.