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. Presented here is a 16th-century lectionary based on the Gospels and Epistles to be used during Sunday services and on feast days throughout the church year, translated from Latin into Nahuatl, possibly by Sahagún or his colleague in Mexico, Father Alonso de Molina, a fluent Nahuatl speaker and author of the first Nahuatl−Spanish dictionary published in the New World. Readings begin with the first Sunday in Advent, and continue through the seasons of the church year, including Epiphany, Lent, the Passion season, Easter, and Pentecost. Readings also are given for celebrating feast days, such as the Purification of the Virgin, the Annunciation, and the Invention (Finding) of the Holy Cross, as well as the feast days of important Catholic saints, such as Sebastian, Mark, and Barnabas. The lectionary features decorated initials and an inscription from a former owner, Phelipe de Baldes (Felipe de Valdez), to Adán Inquaci, an Indian. The document is a key primary source for understanding Sahagún's interpretation of Christianity and the Roman Catholic liturgy for his Nahua audience.