There are several fortune tellers or prophetic figures in Paulo Coelho's The Alchemist; I will assume you are referring to the old Gypsy woman. Santiago is suspicious of her because he has heard that Gypsies engage in criminal activities and are not to be trusted. His encounter with this particular Gypsy, however, forces him to overcome his prejudices and see past stereotypes. He notices that the woman has an icon of the Sacred Heart of Jesus, a symbol typically associated with Catholicism, which seems a sharp contrast to her otherwise pagan practices of divination. Santiago begins to see that one can use follow multiple faiths to see the world in many different ways, but in the end, all faiths are one.