Constantine's Legacy. The Carolingians Series, #1

Raised by his aunt, whose learning made her a very unusual woman, Leonard knows it makes him different, too. His powerful father's duty has kept him from home, so at fifteen Leonard finds it unusual to now ride and learn the art of a warrior with his father, Radulf the Dux Provinciae. An important new duty will take his father and Leonard to Rome where chaos reigns. Three popes will hold power within a period of a few months, all while the Lombard king attempts to take the Church's lands for his own, and the Emperor Constantine V demands the Church regain his lands already conquered by Lombards.
Only the Franks can save the Church, or maybe a lie conceived within the Church.