The Soldier of Gideon by Barry Sadler (Casca Series, Book 20) from Books In Motion.com

Casca Longinus. Cursed by Christ on Golgotha. Condemned to outlive the ages, and wander the globe a constant soldier. Forever fighting and surviving, he must wait for Christ to return. Casca returns to the Middle East, but it is now the twentieth century, and he has signed on to fight a religious war. Outnumbered and poorly equipped, many of the recruits have never seen combat. Casca expects a slaughter. It won't be the first time he has seen his comrades turning the merciless desert sands red with their own blood. But how could the Eternal Mercenary have known that he has joined the most devastatingly efficient army of the modern age? Now, what he imagined to be a slaughter could just end up a victory.