Pope Innocent III convened a general council in 1215 in St. John Lateran comprised of clerics from throughout Western Christendom, a council that would become an object of criticism for the Protestant reformers. Prior to 1215, three previous councils had met in St. John Lateran: Lateran I (1123), Lateran II (1139), and Lateran III (1179). Lateran IV, however, far exceeded its predecessors in both attendance and influence. Over 400 bishops and cardinals attended, but an additional 800 representatives flocked form various religious orders, cathedral chapters, and secular governments. Those attending had neither vote nor voice. All seventy conciliar canons promulgated at Lateran IV were composed by pope and curia, and then pronounced to the gathered clergy and officials.
Innocent III had two express aims in convening the council. First, he sought to renew the crusade in order to take back Jerusalem. Second, he wanted a general reform of the church in head and members. The majority of the seventy canons addressed the latter. The church was struggling at the time to suppress heresy and promote orthodoxy. Cathars in southern France and the Humiliati and Waldensians of northern Italy were unlicensed preachers who largely rejected papal authority and stood at odds with church teaching. Consequently, Lateran IV underscored the role of the bishop in rooting out heresy with his own preaching and by his conscription of others to aid in this service. It also sought to expand limited clergy preparation at cathedral schools, to regulate dress and behavior of priests, and to forbid the establishment of new religious orders outside of one of the monastic rules already existing. Other decrees touched society more broadly, such as the prohibition of consanguinity in marriage to the third degree or restrictions placed upon Jews during Holy Week.
The two most significant canons from the perspective of the Reformation concerned sacraments. First, the doctrine of transubstantiation was defined against the more dualistic view of the Cathars, who rejected any substantial presence of Christ in the sacrament. This position would not only stand until the Reformation, but later to be expanded with the use of Aristotelian metaphysics. Luther rejected transubstantiation in part because he did not agree with the definition and in part because he thought it was an opinion and should not be enforced as dogma. Second, private auricular confession of sins was mandated for all people at least once a year prior to receiving the sacrament. While the canon itself only hoped to encourage more regular confession of sins, it had the result of transforming it into a law imposed upon the faithful and punishable by excommunication. For Luther and the sixteenth-century reformers, this would come to obscure its more evangelical use as a consolation for sinners.