By Thomas Gifford

Bantam. 600 pp. $19.95

For many the Catholic Church presents an enduring and enticing mystery. Hugely powerful and influential, the Holy See appears to represent simultaneously humanity at its polar opposites, an amalgam of the best and worst we have managed.

What else can be said about an institution that encompasses both the Renaissance and the Inquisition, mysticism and reason? To outsiders, the True Church often looks to be lost somewhere in its own catacombs, vaguely medieval and perhaps vaguely sinister.

Fiction has not done much to clear up the mystery. Worldly priests, repressed nuns, chanting monks and the like dot popular novels. Serious authors mine the subject for questions, not for answers. For many writers, the church presents a certain kind of irresistible darkness to place at the center of their work, as they weave their plots around it, shedding no light.

Not much of that central darkness is cleared up in this new novel, "The Assassini," by Thomas Gifford, best known for such thrillers as "The Wind Chill Factor" and "Hollywood Gothic." Instead, like a black hole in space, the central mystery of the church absorbs whatever comes too close, stretching things out of shape, pulling them inexorably toward its center.

Cleverly, Gifford has us consider that greater mystery within the familiar conventions of a lesser one. Like Mary McCarthy in "Cannibals and Missionaries," he uses the genre of the thriller to raise questions that are both pointed and profound, leaving readers perplexed by the issues even as they are wrung out by the adventures. The author manages to bring off an impressively delicate performance that is carefully balanced between ethics and action.

Ben Driskill, lawyer and lapsed seminarian, is the brother of the feisty Sister Valentine and son of Hugh, a hugely rich and powerful lay figure in the church, the sort of man who suspects that papal infallibility may be less reliable than his own. Rumblings on the Driskill estate can make the Vatican tremble.

Suddenly, there is a major earthquake. Sister Val has been nosing into church business and apparently has discovered some dirt. For this the nun is murdered in the family chapel. So, eventually, are lots of others. An elderly priest glides in and out, silently dealing death as if on a sacred mission. There are Nazis abroad. But to Ben Driskill, all these trappings don't matter; it is his sister he must avenge.

Meanwhile, as he sets off to find the killer, the pope is sliding toward death in Rome, and palace intrigues have begun. Of course, we all know there must be a connection, and gradually it emerges. Cardinal D'Ambrizzi, one of the favorites to ascend the throne of Peter, had spent some time with the Driskill family after the war compiling a mysterious document. Hugh had fought in Paris against the occupation, as had the cardinal and other princes of the church. What were they all doing there? Before her murder, Sister Val had secreted a strange photograph for her brother to find, which gives some hints, if only he can figure them out.

As Ben travels about, to Egypt, Paris, Ireland and Rome, he begins to see how deeply the church is involved in the trail of bodies, but how? And what part of the church? Where does the battle for the papal succession fit in? Then he learns of the "assassini."

Dating from the time of the Borgias, these angels of death were dedicated servants of the pope, acting above secular law and apart from church doctrine. Simply put, they did away with papal enemies. But what are they doing in the 20th century? And who is controlling them?

With a few allies, most notably Sister Elizabeth, a colleague and friend of Val's (with whom Ben soon falls in love), and Father Artie Dunn, a cheerful and worldly novelist in clerical garb, Driskill looks for answers. Several more bodies pile up in his wake before he finds them: a suicide in Egypt, a murder in Paris, a crucifixion in Ireland. But gradually, the circle narrows as Driskill uncovers a squalid nest of plot, intrigue and power politics of the most petty sort that make "the Reichs Chancellory or the Supreme Soviet look like a child's idea of the real stuff."

But this hive of machination lies in the Catholic Church, the most powerful instrument of faith on the planet. What is going on here? This is the largest question of all, the one Gifford makes us ponder throughout his novel.

God, who here often seems out of place in His own house, sure does work in mysterious ways. That His wonders do get performed despite their earthly interpreters is perhaps the greatest miracle of all.

The reviewer teaches at Marist College and frequently reviews Latin American and other fiction.