The Gemini Contenders book by Robert Ludlum

1939, Salonika Greece: In the dead of a December night, a clandestine order of monks desperately transport a mysterious vault to a hiding place high in the Italian Alps. The trip is planned by a secret benefactor. The monks will murder and suicide to keep their secret. The sinister cache, concealed for centuries, could rip apart the Christian world.
Decades pass: The order, killer Nazis, governments, and assassins chase the Italian magnate and family across continents.
1970ish: Dying father assigns search to his twin sons. The liberal lawyer and killer soldier oppose each other to the death. Murders follow them from Vietnam and American States to the mountains of Italy.