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

The horseman in Paulo Coelho's The
Alchemist 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.

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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 foreshadowing.) 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 danger—in 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:

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Be careful with your href="http://dictionary.reference.com/browse/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 was—which is why he seemed so threatening—for 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:

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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.