It's a lonely life that the elves have adopted for themselves.
I wonder why nobody remembered him? If he was a truly influential person, there should be statues and records of him, yet there is nothing that they knew of, even to Frieren who has lived over a millennium. Would that mean that he has lived for so long that any and every evidence of his achievements have wasted away and reduced to dust? But then did he "recently" adopt his faith for the goddess then, as i do not believe the religion of humankind was created that long ago. We know that it couldnt be based on the truth either, because he never saw the goddess herself, so she never actually appeared.
I guess after living for so long, watching entire generations of people dying and seeing yourself be forgotten in the sand of time can be an invitation to madness, and keeping faith that someone outlives you and remembers you is needed to keep on going.