“Unfortunately, not all can be permitted to possess immortality. Too many would make immortality miserable or hellish for the rest, and they would try to control others through their control of the resurrection machinery.”

The Riverworld series, Gods of Riverworld (1983)
Context: The truth is that you can be immortal, relatively so, anyway. You won't last beyond the death of the universe and probably not nearly as long as the universe does. But you have the potentiality for living a million years, two, perhaps three or more. As long as you can find a Terrestrial-type planet with a hot core and have resurrection machinery available.
Unfortunately, not all can be permitted to possess immortality. Too many would make immortality miserable or hellish for the rest, and they would try to control others through their control of the resurrection machinery. Even so, everybody, without exception, is given a hundred years after his Earthly death to prove that he or she can live peacefully and in harmony with himself and the others, within the tolerable limits of human imperfections. Those who can do this will be immortal after the two projects are completed.