From Pope St. John Paul II’s Apostolic Exhortation Reconciliatio et Pænitentia (1984):
Over the course of generations, the Christian mind has gained from the Gospel as it is read in the ecclesial community a fine sensitivity and an acute perception of the seeds of death contained in sin, as well as a sensitivity and an acuteness of perception for identifying them in the thousand guises under which sin shows itself. This is what is commonly called the sense of sin.
This sense is rooted in man’s moral conscience and is as it were its thermometer. It is linked to the sense of God, since it derives from man’s conscious relationship with God as his Creator, Lord and Father. Hence, just as it is impossible to eradicate completely the sense of God or to silence the conscience completely, so the sense of sin is never completely eliminated.
Nevertheless, it happens not infrequently in history, for more or less lengthy periods and under the influence of many different factors, that the moral conscience of many people becomes seriously clouded. “Have we the right idea of conscience?” I asked two years ago in an address to the faithful. “Is it not true that modern man is threatened by an eclipse of conscience? By a deformation of conscience? By a numbness or ‘deadening’ of conscience?”97 Too many signs indicate that such an eclipse exists in our time. This is all the more disturbing in that conscience, defined by the [Second Vatican] council as “the most secret core and sanctuary of a man,”98 is “strictly related to human freedom. . . . For this reason conscience, to a great extent, constitutes the basis of man’s interior dignity and, at the same time, of his relationship to God.”99 It is inevitable therefore that in this situation there is an obscuring also of the sense of sin, which is closely connected with the moral conscience, the search for truth and the desire to make a responsible use of freedom. When the conscience is weakened the sense of God is also obscured, and as a result, with the loss of this decisive inner point of reference, the sense of sin is lost. This explains why my predecessor Pius XII, one day declared, in words that have almost become proverbial, that “the sin of the century is the loss of the sense of sin.”100
97 Pope John Paul II, Angelus Message of March 14, 1982: Insegnamenti V, 1 (1982), 861.
98 Gaudium et Spes, 16.
99 Pope John Paul II, Angelus Message of March 14, 1982: Insegnamenti V, 1 (1982), 860.
100 Pope Pius XII, Radio Message to the U.S. National Catechetical Congress in Boston (October 26, 1946): Discorsi e Radiomessaggi VIII (1946) 288.
— Reconciliatio et Pænitentia 18