The Midas Coffin(1975)
(The fifth book in the Inquisitor series)
A novel by Simon Quinn (Martin Cruz Smith)
His name is Francis Xavier Killy. He's a lay brother of Militia Christi working for the Holy Office of the Inquisition in Rome. Unbound by vows of celibacy and fears of common man, he's tough, aggressive, and a master of highly original deadly arts. Part saint, part sinner, he kills mercifully quick.
Used availability for Martin Cruz Smith's The Midas Coffin