Why does the alchemist tell Santiago the story of of the two sons in Paulo Coelho's The Alchemist?
[As with any piece of art, it can be interpreted based upon how it speaks to each person that comes in contact with it: this is only my interpretation.]
In Paulo Coelho's The Alchemist, the alchemist tells Santiago about two sons. For one son it appears as if he will be long remembered because he is a great poet. However, when his emperor (Tiberius) dies, the first son's poetry is eventually forgotten. Ironically, it is the second son who is long remembered—and an unlikely candidate. He is a Roman centurion. His servant is sick and so he personally rides out to a well-known "rabbi" (Jesus) to ask if the rabbi might heal his servant.
Along the way, he learned that the man he was seeking was the Son of God. He met others who had been cured by him...And so, despite the fact that he was a Roman centurion, he converted to their faith.
The centurion expresses his humility in the presence of the rabbi, as well as his strong belief that if this great man will only wish it to be so, his servant will be well again. Christ praises the man for his great faith, and the story of the centurion is told over and again in the Scriptures—in the New Testament of the Bible.
The alchemist notes:
No matter what he does, every person on earth plays a central role in the history of the world. And normally he doesn't know it.
In saying this, the alchemist is not only alluding to the importance of the centurion's faith in teaching others and setting an example that has been referred to countless times down through the ages, but he also speaks of how any person and every person can make an enormous difference in the world and not even be aware of it.
This provides Santiago with the knowledge that he has the capacity to do great things that will change the face of the world, and that simply by being himself and living his Personal Legend he can do so: it is that easy.