From Pope St. John Paul II’s Apostolic Exhortation Reconciliatio et Pænitentia (1984):
In order to understand sin we have had to direct our attention to its nature as made known to us by the revelation of the economy of salvation: This is the mysterium iniquitatis. But in this economy sin is not the main principle, still less the victor. Sin fights against another active principle which—to use a beautiful and evocative expression of St. Paul—we can call the mysterium or sacramentum pietatis. Man’s sin would be the winner and in the end destructive, God’s salvific plan would remain incomplete or even totally defeated, if this mysterium pietatis were not made part of the dynamism of history in order to conquer man’s sin.
We find this expression in one of St. Paul’s pastoral letters, the First Letter to Timothy. It appears unexpectedly, as if by an exuberant inspiration. The apostle had previously devoted long paragraphs of his message to his beloved disciple to an explanation of the meaning of the ordering of the community (the liturgical order and the related hierarchical one). Next he had spoken of the role of the heads of the community, before turning to the conduct of Timothy himself in the church of the living God, the pillar and bulwark of the truth.” Then at the end of the passage suddenly, but with a profound purpose, he evokes the element which gives meaning to everything that he has written: “Great indeed, we confess, is the mystery of our religion.”104
Without in the least betraying the literal sense of the text, we can broaden this magnificent theological insight of St. Paul into a more complete vision of the role which the truth proclaimed by him plays in the economy of salvation: “Great indeed,” we repeat with him, “is the mystery of our religion,” because it conquers sin.
104 1 Tm 3:15f.
— Reconciliatio et Pænitentia 19