Mysterium fidei (encyclical)

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Mysterium fidei Latin for 'The Mystery of Faith' Encyclical of Pope Paul VI
Signature date	3 September 1965
Subject	On the doctrine and worship of the Holy Eucharist
Number	3 of 7 of the pontificate
Text	In Latin In English
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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 Pope Paul 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 Ignatius of Antioch, referring to the Blessed Sacrament the "medicine of immortality". The Pope acknowledged that there were many "real" presences of Christ, but that in the Communion bread this presence is real and "substantial".

The letter, however, received little attention as the world's interest was focused at the time on the final works of the council fathers, particularly Lumen gentium, issued in November of the same year at the conclusion of the Council.