Earlier in the story, the alchemist told Santiago “when you possess great treasures within you, and try to tell others of them, seldom are you believed.” At the end of the story, how did this simple lesson save Santiago’s life? How did it lead him back to treasure he was looking for?

At one point on Santiago's quest for his Personal Legend, he and the alchemist are travelling together toward the Pyramids. They encounter some armed Arabs that stop the alchemist and Santiago. The robbers demand to know what Santiago and the alchemist are doing and where they are going. The alchemist has items that the other men simply do not understand, and the alchemist's answer mystifies the men.

"What are these things?" he asked.
"That's the Philosopher's Stone and the Elixir of Life. It's the Master Work of the alchemists. Whoever swallows that elixir will never be sick again, and a fragment from that stone turns any metal into gold."

The Arabs' response is to laugh at the answer, and the men then leave. Santiago immediately asks why the alchemist did that, and the alchemist responds with the quote in question.

"Are you crazy?" the boy asked the alchemist, when they had moved on. "What did you do that for?"
"To show you one of life's simple lessons," the alchemist answered. "When you possess great treasures within you, and try to tell others of them, seldom are you believed."

The alchemist's answer may have saved Santiago's life, because the men figured Santiago and the alchemist were crazy and carrying nothing but worthless junk. Had the men believed that Santiago was carrying anything of value, they probably would have killed Santiago and the alchemist and taken the stuff. The alchemist's quote calls special attention to a person having great treasures within. Santiago has been on a treasure hunt for an actual, tangible treasure. He's hoping for a big monetary payoff. But Santiago will eventually learn that the treasure he is looking for has been within all along. It is love, and Fatima, and how that emotion has always been within him. Santiago just needed the right key to unlock that treasure, and that particular key was the journey itself.

"No," he heard a voice on the wind say. "If I had told you, you wouldn't have seen the Pyramids. They're beautiful, aren't they?" . . .
The wind began to blow again. It was the levanter, the wind that came fromAfrica. It didn't bring with it the smell of the desert, nor the threat of Moorish invasion. Instead, it brought the scent of a perfume he knew well, and the touch of a kiss—a kiss that came from far away, slowly, slowly, until it rested on his lips.
The boy smiled. It was the first time she had done that.
"I'm coming, Fatima," he said.

While Santiago digs, trying to find the treasure, he is approached by refugees who rob him of the gold coins given to him by the alchemist. The robbers believe that Santiago was trying to hide money in the sand and they force him to dig it up. In the early morning, the robbers beat Santiago up. Santiago yells, telling them that he was digging because he had a dream that there was buried treasure where he was digging. As per the alchemist's advice, people tend to not believe you when you tell them about something grandiose. The leader tells Santiago that he shouldn't be so stupid because he also had a dream about where a treasure was buried, except he did not go searching for it. Santiago now knows the location of the treasure because it is revealed through the dream of the leader of the refugees.
The advice the alchemist gives also saves Santiago's life in another way. Santiago realizes that the journey itself and the people he met along the way were more valuable than the physical treasure he found.