Fray Bernardino de Sahagún was a Franciscan missionary who arrived in Mexico from Spain in 1529 and stayed until his death in 1590. He worked with the indigenous peoples of the area to document their cultures and religions, in large part motivated by the conviction that better understanding of their beliefs and practices would improve the efforts to convert them to Christianity. His methods have led some scholars to consider him the first ethnohistorian, and he is remembered today as much for his ethnographic and linguistic documentation of the Nahua peoples and Aztec civilization as for his missionary work. This work is a compilation of sermons in the Nahuatl language by Sahagún, begun in 1540, and revised and corrected in 1563, part of his “Postilla,” or scriptural commentaries, used in missionary work among the Aztecs. The cycle of Sunday sermons for the church year is incomplete, beginning with the first Sunday in Advent, and concluding with the 19th Sunday after Pentecost and a single sermon for the last Sunday before Advent. The cycle of sermons for saints' days, the "Santoral," is also incomplete, containing sermons in honor of only a few saints (Andrew, Thomas, Stephen, and John). The manuscript includes marginalia in Sahagún's hand, such as comments, additions, and explanations, and is on the native maguey paper also used to create Aztec codices. It is a key primary source for understanding Sahagún's interpretation of Christianity for his Nahua audience.