Set two thousand years in the future, Cestus Dei describes a galaxy in which the various worlds settled by the human race are bound to Earth, not by political structures, but by religion. Effectively, each religion ends up with its own interstellar empire, and the supreme decision-making body in humanity is the United Faiths.
Some centuries in the past, civilisation became corrupt and went into decline, and whole star clusters broke away. The Flavian System is one such group, which has become a sort of replay of the late Roman Empire, with slavery and gladiatorial combat, but also featuring starships, ray guns, and genetically engineered "constructs", which serve as gladiators, bodyguards, and other menials.
When the Flavian System is rediscovered, Father Miles, a Jesuit priest from the order named in the title, is dispatched to return the Flavian System to the Roman Catholic fold before it is snapped up by the Medina Caliphate or the Sanhedrin of the Third Temple. The creation of constructs, or the "soulless", is an affront to all faiths, and since the Flavian System has largely slipped back into Paganism, it could be argued that it's no longer the responsibility of the Church, and thus forcibly converting the inhabitants to a faith – any faith – would be a public service.
Miles, whose name means "soldier" in Latin, is a kind of cross between James Bond, Rambo, Machiavelli, and Dirty Harry: he's competent and dangerous, especially in hand-to-hand combat, has a keen grasp of political reality, and a disturbingly elastic attitude towards the rules under which he is supposed to be operating.