A prehistoric man encounters an alien spaceship. The aliens decide, for whatever reason, to make the man immortal. The novel describes all the lives the man has had; Inca king, Viking, modern man. He describes how he has to move from region to region to hide his secret.
I believe this is Ben Bova's series, Orion, about an immortal cave man, following his defense of Humanity through the ages. There are six books in the series.
John O'Ryan is not a god...not exactly. He is an eternal warrior destined to combat the Dark Lord through all time for dominion of the Earth. Follow him, servant of a great race, as he battles his enemy down the halls of time, from the caves of our ancestors to the final confrontation under the hammer of nuclear annihilation.
It is possible that you are looking for In the Face of My Enemy by Joseph Delaney, published in 1985.
Eighteen thousand years ago a priest ascended a high place, there to hold converse with his gods. That night the gods chose to bless him with their physical reality. Alas, mortal flesh cannot bear the presence of godhead, and so he died.
And was reborn. But ever after he was not as other men. He could change his form at will, his wounds healed instantly. . . and he did not age.
As millennia passed he grew very wise, and came at last to understand. His destiny lay not among men, but among the stars.