I see many discussions on the internet that haven't quite got a clue about what AC Clarke did in CHILDHOOD'S END.
Karellen is the saddest person in the universe and he is not Satan but he is Lucifer, the brightest angel in the heavens.
In AC Clarke's universe, every race of sentient beings will ultimately evolve into non-corporeal beings of thought. This has already happened and in some cases the result was disastrous. A world and race of beings can self destruct as it attempts the ascension to higher being. However, there were sufficient successes to form the Overmind. Contact with the Overmind at the time a race evolves into non-corporeal beings of thought isn't desirable. I don't know why off-hand. In a general sense it comes down to the question of how can mere mortals see the mind of God but that isn't altogether correct.
However, there is a one race or maybe two who cannot make that final step and evolve into non-corporeal beings of thought. In AC Clarke's universe, this is the race of Karellen and the Overlords. This race pledge to help all the other races of the Universe ascend into what they cannot.
So like all good children leave their parents when they grow up, the children of the human race, leave their parents behind and evolve into the substance of what might be called "god" in the broadest sense of the concept.
This is always painful for parents. When a child is born its parents have extravagant dreams of success for their child. Everything is possible. our newborns will be the next Einsteins, Mozarts, Clarke Gables, Lady Gaga's or U Thant, or Nelson Mandela, or Mother Theresa. Instead, As the child grows, he or she breaks the hearts of its parents by making choices. Most people live mundane lives. As bad as the pains of child rearing are, none of us would give it up.
But the Overlords cannot follow. They watch. They assist. They are faithful agents of the Overmind.
AC Clarke explains that every race of beings knows in the depths of their minds, in the tiny parts of their beings-- that the final step of evolution is into the Overmind. Deep inside we know that when the winged, red-skinned, horned-beasts appear, our lives are at an end in this world. We fear the unknown step and demonize it.
In the movie version, Karellen offers Ricky Stormgren a life with his first love in a fake hotel room. This is Karellen's justification of his own existence. His existence as the one who watches. The one who assists. The one who cannot follow but lives in the shadow of paradise. He is Lucifer, the brightest star of the heavens. A tragic figure. That is the life he offers Ricky Stormgren who in a very human act, returns to his current love to die in her arms.
This is pointed out a second time as Milo Roddicks delivers the last words from the changing earth before it is consumed in the evolution of man... Roddick's asks if "anyone else invented Cookie Dough Ice Cream?" And as a last request, Roddicks asks for something to be left behind. That music you hear is familiar to many -- Ralph Vaughan Williams THE LARK ASCENDING.
Listen to it:
https://www.youtube.com/watch?v=3b0rN43q6jo
And Karellen agrees. For all who pass this way, something is left of what was...