While the Pope's battalions wreak havoc on the Ousters, the agents of the Inquisition close in on Aenea. But despite its galaxy-spanning power, the Church has reason to fear two groups of enemies: the Ousters, gene-altered humans whose idiosyncratic space habitats owe allegiance to no one, and the scattered followers of a young girl named Aenea, who is rumored to be a new messiah and whose most dangerous gift, quite literally, is empathy. This time the Church does not have to beg support from temporal powers its own battle fleet is the biggest, fastest and best armed in space. The significance of the Pontiff's new name is not lost on the more learned members of the hierarchy: two millenniums earlier, another Pope Urban called for a holy war against unbelievers, a preaching that led directly to the First Crusade. ''The Rise of Endymion'' opens with the ninth death of Pope Julius XIV and his rebirth as Urban XVI. Those who ''wear the cruciform'' may be killed by accident or illness, but unless the very atoms of their body have been blown apart, the dead can be resurrected with all memories and faculties intact. In the Year of Our Lord 3131, most humans in the galaxy are in communion with the reborn Church of Rome, which controls access to the life-giving parasite known as the cruciform. But taken together, these four volumes represent one of the finest achievements of modern science fiction, a convincing demonstration of how liberating, in the hands of a masterly practitioner, genre conventions can be. The current book and its forerunner, ''Endymion,'' follow the earlier volumes so closely in plot development that I am not sure how comprehensible they would be to readers unfamiliar with the rest of the series. In reviewing the first two books in this series, ''Hyperion'' and its sequel ''The Fall of Hyperion,'' I noted that they actually constituted a single thousand-page novel that should be read from the beginning. Yet ''The Rise of Endymion,'' like its three predecessors, is also a full-blooded action novel, replete with personal combats and battles in space that are distinguished from formulaic space opera by the magnitude of what is at stake - which is nothing less than the salvation of the human soul. No one in modern science fiction, not even Wolfe, has dealt more sensitively with the interface between religion and science.

This is the animating premise of THE RISE OF ENDYMION (Spectra/Bantam, $23.95), the fourth book of Dan Simmons's galaxy-spanning series that began in 1989 with ''Hyperion.'' For vastness of scope, clarity of detail and seriousness of purpose, Simmons's epic narrative is on a par with Isaac Asimov's Foundation series, Frank Herbert's ''Dune'' books, Gene Wolfe's multipart ''Book of the New Sun'' and Brian Aldiss's Helliconia trilogy.

Rise of endymion