God of Death by Barry Sadler, (Casca Series, Book 2) from Books In Motion.com

He was cursed by Jesus on Golgotha, condemned to immortality and the life of a soldier, until the Second Coming. Galley slave and gladiator, warrior and vagabond, Casca travels the world living by his wits and skills as a fighting man. Waiting, knowing he cannot die, he fears discovery. His travels take him north to the land of the Vikings. Then west to the Toltecs. In Mexico atop a storm shrouded pyramid, priests pepare to offer Casca in sacrifice. The knife plunges, but the man who cannot die rises from the altar. Reclaiming his still beating heart he proclaims himself the God of Death.