"This catechism is based on the encyclical letter MYSTERIUM FIDEI published by Pope Paul VI on the feast of Pope St. Pius X, the third of September 1965. It is one of the most unusual letters of a Vicar of Christ. It was issued during the sessions of the Second Vatican Council. The reason for its publication, as the Pope explained, was to ensure that the liturgical modifications introduced by Vatican II would bear fruit for the sanctification of souls. He feared that the rise of errors regarding the Real Presence already evident then would prevent the revised liturgy from bearing supernatural fruit. He was concerned that great harm would be done to the Christian faithful. Needless to say, his fears have been fully justified. This catechism faithfully presents the teachings of MYSTERIUM FIDEI and also draws on the teachings of both Pope Paul VI and his successor Pope John Paul II." –Forward by Rev. John A. Hardon, S.J. Paper.