Although he wrote sermons, letters, and commentaries on Holy Scripture, Lombard's Four Books of Sentences (1148-51) established his reputation and subsequent fame, earning him the title of magister sententiarum ("master of the sentences").
The Sentences, a collection of teachings of the Church Fathers and opinions of medieval masters arranged as a systematic treatise, marked the culmination of a long tradition of theological pedagogy, and until the 16th century it was the official textbook in the universities.
Hundreds of scholars wrote commentaries on it, including the celebrated philosopher St.
Thomas Aquinas.Book I of the Sentences discusses God, the Trinity, divine guidance, evil, predestination.
While Lombard showed originality in choosing and arranging his texts, in utilizing different currents of thought, and in avoiding extremes, of special importance to medieval theologians was his clarification of the theology of the sacraments.
He asserted that there are seven sacraments and that a sacrament is not merely a "visible sign of invisible grace" (after Augustine of Hippo) but also the "cause of the grace it signifies." In ethical matters, he decreed that a man's actions are judged good or bad according to their cause and intention, except those acts that are evil by nature.