Along with this extremely rigid theory, Catholicism has always had a certain degree of toleration for what it held to be sin. The Church has recognized that ordinary human nature could not be expected to live up to its precepts, and has been prepared to give absolution for fornication provided the sinner acknowledged his fault and did penance. This practical toleration was a method of increasing the power of the clergy, since they alone could pronounce absolution, and but for absolution fornication would entail eternal damnation.
Source: Bertrand Russell :Marriage and Morals, 1929
More info.: https://russell-j.com/beginner/MM05-090.HTM