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 the Blessed Sacrament the "medicine of immortality."
The letter, however, received little attention as the world's interest was focused at the time was on the final works of the council fathers, particularly Lumen Gentium, issued in November of the same year at the conclusion of the Council.
Paul VI felt very strongly that certain theological currents were threatening several Catholic doctrines and practices including:
Pope Paul VI in the opening of the letter declares the following teachings are impermissible:
"These and similar opinions do great harm to the faith and devotion to the Divine Eucharist. And therefore, so that the hope aroused by the Council, that a flourishing of eucharistic piety which is now pervading the whole Church, be not frustrated by this spread of false opinions"