In Paulo Coelho's The Alchemist, what advice does the horseman give to Santiago?

The
horseman in 's is actually the alchemist that Santiago has been searching
for. However, when the boy (Santiago) first sees the rider come pounding up to him, he has no
idea of the man's real identity.

The first thing the horseman demands to know
is the identity of the person who has dared to read the omen of the fighting hawks.

Suddenly he heard a thundering sound, and he was thrown to the
ground by a wind such as he had never known...Before him was an enormous white horse, rearing
over him with a frightening scream...

(The presence of
the wind here is .) Santiago gets his first glimpse of the horseman who has all but trampled him
with his animal. The man is dressed completely in black, with a falcon resting on his arm.
Drawing his sword, the man speaks.

"Who dares to read
the meaning of the flight of the hawks?" he demanded so loudly that his words seemed to
echo...

Santiago does not hesitate, but identifies
himself as one who has "dared to do so." The boy is prepared to die by this sword if
he has done something to offend, but calmly explains that he has saved lives by doing so. Rather
than killing Santiago, the horseman asks why he read the omen of the birds. Santiago answers
that he "read" what the birds wanted him to know, which was that
the oasis was in dangerin this way, he could save the people there. The man in black then
asks:

Who are you to change what Allah has
willed?

The boy explains that Allah has created all
things, as well as the birds who gave him the message. The horseman lowers his sword but warns
Santiago:

Be careful with your href="https://www.dictionary.com/browse/prognosticate"
title="prognosticate">prognostications...When something is written, there
is no way to change it.

Santiago says that he had a
vision, and then tells the man that he is following his Personal Legend, thinking that this man
could not possibly understand what that is. The man
does know (and so much more that Santiago could never imagine at that
time), and points out that he had to see how brave Santiago waswhich is why he seemed so
threateningfor bravery is absolutely necessary if the boy is to understand the "Language of
the World."

The man's final advice to Santiago is that he must not give
up on his quest. Then he warns him:

You must love the
desert, but never trust it completely. Because the desert tests all men: it challenges every
step, and kills those who become distracted.

Santiago now
knows he must be more vigilant than ever. The man in black tells Santiago that "if your
head is still on your shoulders at sunset," the boy should find him. The young man asks
where the horseman lives, and the direction in which the man points lets Santiago know he has
finally met the alchemist.