Mysterium Fidei, a Latin phrase meaning mystery of faith or mystery of the faith, is a Christian theological term for an article of faith or doctrine which defies man’s ability to grasp fully. Examples such as the Trinity are mentioned in the Catechism of the Catholic Church, 1066-1068.
In the Tridentine Mass the people do not hear the phrase "mysterium fidei", because the priest speaks the words of the Canon of the Mass inaudibly.
Since the revision of the Roman Missal that was promulgated officially in 1969 and was actually published in 1970, the words "mysterium fidei" have been moved to immediately after the consecration, the showing of the chalice to the congregation and the priest's genuflection. The priest celebrating the Mass proclaims the words, and the people respond with one of the Memorial Acclamations. The same position is assigned to the phrase also in the newer Eucharistic Prayers of the Roman Rite.
Thus, the corresponding text in the Roman Canon or First Eucharistic Prayer is now as follows (here accompanied by the reductive 1973 ICEL translation, due to be replaced soon with a more exact translation):