Starting with the 4th century, the influence of monasticism over penitence has imposed itself on a large scale in the Byzantine Empire. Spiritual fathers were encountered especially in the monasteries and in the desert, guiding the spiritual lives of young monks and oftentimes those of laymen as well, who would go to these gifted men, carriers of Holy Spirit, in order to ask for advice and even to receive the forgiveness of their sins. Thus, even though they had retreated from society, looking for solitude, the fathers of the desert were great missionaries for those who had embraced monastic life and for all Christians alike. These missionaries, difficult to equal, of the church, were sought because they had “the gift of word” and the Godly gift of seeing the state of the soul of each person, just as we see the faces of people, in other words the gift of cardiognosis or knowing of the hearts. That is why their advice or words of strengthening were received by all- monks or laymen- as from God. The fathers reveal the fact that the beginning of sin lies inside the mind, in the fiery thoughts instilled by demons, for these avert the mind from prayer, contemplation and seeing God. In the present study we can read about both the demonic thoughts that torment the man and the remedies for what these cause in anthological pages of ascetic psychology and demonology.