The Fourth Lateran Council (1215) ruled on a number of issues related to canon law (rules of the church's order and operation). Among these was a firm affirmation of the importance of regular confession by the laity to the parish priest. Though sacramental confession was practiced before this council, the Fourth Lateran Council lent this strong (but not always consistent) practice the weight of the Church's full authority.
CANON 21
SUMMARY: Everyone who has attained the age of reason is bound to confess his sins at least once a year to his own parish pastor with his permission to another, and to receive the Eucharist at least at Easter. A priest who reveals a sin confided to him in confession is to be deposed and relegated to a monastery for the remainder of his life.
Text. All the faithful of both sexes shall after they have reached the age of discretion faithfully confess all their sins at least once a year to their own (parish) priest and perform to the best of their ability the penance imposed, receiving reverently at least at Easter the sacrament of the Eucharist, unless perchance at the advice of their own priest they may for a good reason abstain for a time from its reception; otherwise they shall be cut off from the Church (excommunicated) during life and deprived of Christian burial in death. Wherefore, let this salutary decree be published frequently in the churches, that no one may find in the plea of ignorance a shadow of excuse. But if anyone for a good reason should wish to confess his sins to another priest, let him first seek and obtain permission from his own (parish) priest, since otherwise he (the other priest) cannot loose or bind him.
Let the priest be discreet and cautious that he may pour wine and oil into the wounds of the one injured after the manner of a skilful physician, carefully inquiring into the circumstances of the sinner and the sin, from the nature of which he may understand what kind of advice to give and what remedy to apply, making use of different experiments to heal the sick one. But let him exercise the greatest precaution that he does not in any degree by word, sign, or any other manner make known the sinner, but should he need more prudent counsel, let him seek it cautiously without any mention of the person. He who dares to reveal a sin confided to him in the tribunal of penance, we decree that he be not only deposed from the sacerdotal office but also relegated to a monastery of strict observance to do penance for the remainder of his life.