The Eucharist is a very great mystery; in fact, in the words of the Holy Sacrifice of the Mass, the Eucharist is the mystery of faith.

In this encyclical letter Mysterium Fidei, Pope Paul VI hopes for a new wave of Eucharistic devotion to sweep over the Church. He also seeks to dispel false opinions on the Eucharist that had arisen at the time the encyclical was promulgated in 1965 during the Second Vatican Council. This document later provided the foundation for the Council's historic Constitution on the Sacred Liturgy.

"The Mystery of Faith, that is, the ineffable gift of the Eucharist that the Catholic Church received from Christ, her Spouse, as a pledge of His immense love, is something that she has always devoutly guarded as her most precious treasure," Paul VI writes, "for there is nothing more consoling here on earth, nothing more efficacious for progress along the paths of holiness."