23 Certainly We know, and with you, Venerable Brethren, We deplore the fact that in our day the idea and the name of expiation and penance have with many lost in great part the power of rousing enthusiasm of heart and heroism of sacrifice. In other times they were able to inspire such feelings, for they appeared in the eyes of men of faith as sealed with a divine mark in likeness of Christ and His Saints: but nowadays there are some who would put aside external mortifications as things of the past; without mentioning the modern exponent of liberty, the "autonomous man" as he is called, who despises penance as bearing the mark of servitude. As a fact the notion of the need of penance and expiation is lost in proportion as belief in God is weakened, and the idea of an original sin and of a first rebellion of man against God becomes confused and disappears.
Source: Caritate Christi Compulsi (Vatican.va)