The Warrior by Barry Sadler (Casca Series, Book 17) from Books In Motion.com

Wandering the globe under a curse received from the dying Christ on Calvary, Casca Longinus fights onward, surviving and looking for the day when He promised to return, so the curse may end. Now, shipwrecked on a South Seas island inhabited by warrior tribes, Casca quickly proves himself fearless in combat. Wielding spear and club with murderous precision, the Eternal Mercenary soon earns a respected place in his adopted tribe's barbarously primitive rituals. Then, when he unveils the devastating potential of a new and magical weapon, Casca is elevated from warrior to god, and summons his army of believers for the bloodiest of the island's battles.