In Rome, the area of the human relationship with the deity was the framework in which appeared and developed the concept of mutual obligation, fides. On the one hand, this meant carefully observing the ancestral religion and fulfilling the promises made to the deity; on the other hand, fides was also a moral trait of the person in question. To be of good faith means to show loyalty, sincerity, honesty; to obey your word, to keep your promise. Subsequently, the term in question was transformed into a legal concept, becoming bona fides. Initially used in the Republican era, we find the concept of good faith in the oldest sources of classical civil law, such as the Perpetual Edict and the Institutions of Gaius and we find it applied even in imperial law, proof being Corpus iuris. It will be studied in the faculties of the medieval era and will receive a place of honour in Western civil law. Probability, diligence, lawlessness and abstention from the harm of another (alterum non laedere), as component elements of bona fides, have translated, in the legal plane, that honest and direct life taken from the Stoics and has been perpetuated in contemporaneity as an indispensable principle of law.