The Temple and the Stone by Katherine Kurtz

The Knights Templar was a military order founded during the time of the crusades to protect pilgrims traveling to the Holy Land. Legend endows the Templars with magical powers with which they are said to have altered the course of history. "The Temple and the Crown" picks up in 1306 with the crowning of Robert Bruce in Scotland. Bruce immediately faces a challenge to his throne, and Pope Clement and King Philip of France, jealous of the Knights' magical powers, wealth, and charm, have them arrested on trumped-up charges of black magic, blasphemy, and consorting with the Devil. The Templars' only hope is to flee as fugitives and seek a new home...and a safe haven for the mystical treasures they guard.