Mysterium Fidei is an encyclical letter of Pope Paul VI on the Eucharist, published in September 1965. Mysterium Fidei was issued just as the closing session of the Second Vatican Council was beginning. Written in a stern and troubled tone, its purpose was to counter certain theological movements which he perceived were gaining ground in the Roman Catholic Church. Using terminology such as "pastoral concern" and "anxiety," the letter sends a direct and unequivocal message to the Church regarding the Eucharist. The Pope clearly feared that these novel teachings were threatening the Eucharistic piety which had marked the Catholic Church since the earliest centuries. To emphasize the centrality of the Eucharist in the Church, the Pope echoed the words of St. Ignatius of Antioch, referring to