In the twentieth year of his pontificate, on September 14, 1998, on the Exaltation of the Holy Cross, Pope John Paul II reads the Encyclical Letter on the Relationships between Faith and Reason “Fides et ratio”. The concept of “Christian philosophy” takes on meaning and receives new contributions to the definition of the phrase. Chapter IV of the Encyclical Letter emphasizes the significant stages of the encounter between faith and reason, the drama of the separation between the two, and the eternal novelty of St. Thomas’ thought. Christian philosophy can only be a realistic philosophy, and as philosophy approaches moderate realism, it is Christian. A philosophy of the self has nothing to do with the Christian philosophy. Many consider philosophy to be something indifferent to Christianity and to Christians; they do not want to understand that the meaning and fate of Christianity stands or falls with the philosophy that adopts it; and because apart from Thomism, the other philosophies do not agree with Christianity, without compromise on one side or the other, they say: not only in reality, but also in theory, Thomism is Christian philosophy; the others are more or less philosophical, because they are more or less Christian. The “dark cloud” of mystery that separates believers from unbelievers will help to develop a balanced and communicable level of understanding Christian philosophy, useful to both. In this first part we will focus on: the presence of St. Paul in Athens; the cautious attitude of Christians towards gnosis; the role of Saint Irenaeus and Tertullian.