Shamseddin Mohammad Hafez Shirazi(1390 d.) is one of the greatest poet of Persia in the medieval era. He is the most beloved poet by the Iranians. 『Tongue of Mystery』translated into Korean is Hafez``s selected poems and contains 88 ghazals. As a matter of fact, the Korean reader is not familiar with the Islamic literature. Indeed this book is not easy to translate, but it is not easy for the reader to understand. The aim of this paper is to solve out the difficulty, in particular the understanding of religious terms. The general recognition on the Iran(Persia) is solely considered as Islamic country. Before the advent of Islam, Persia had had the traditional beliefs of its own. The low understanding of this book is ascribed to the reason for the shortage of recognition on the Persia``s ancient religions, connected to the Mithraism-Zoroastrianism-Buddhism-Sufism. Persian literature in the medieval era had been laid on the foundation of Persian Sufism. In this paper the definition and formation process of Sufism is newly introduced through the Persian documents. Sufism is the Persian``s philosophy and Persia is the home of Sufism that accounts for the important fields in humanities. Thus the correct understanding of Sufism enables us to appreciate the medieval poetry in Persia.