The Eternal Mercenary by Barry Sadler (Casca Series, Book 1) from Books In Motion.com

When they flew Casey into the hospital at Nha Trang, the medics were sure he would die. That he didn't was only the first surprise. The second, and bigger one, was that Casey had been fighting for two thousand years, ever since that day on Golgotha when he thrust his lance into the side of the man on the cross. ""Soldier, you are content with what you are. Then that you shall remain until we meet again."" Thus does Casca's journey begin, a man who cannot die, does not age, and knows no skills but those of battle.