7 Our previous Encyclicals were directed to throwing the light of Catholic doctrine upon the gravest of the problems peculiar to modern life. Our present Encyclical finds a natural place among these others, opportunely supplementing them. The priest is, indeed, both by vocation and divine commission, the chief apostle and tireless furtherer of the Christian education of youth; in the name of God, the priest blesses Christian marriage, and defends its sanctity and indissolubility against the attacks and evasions suggested by cupidity and sensuality; the priest contributes more effectively to the solution, or at least the mitigation, of social conflicts, since he preaches Christian brotherhood, declares to all their mutual obligations of justice and charity, brings peace to hearts embittered by moral and economic hardship, and alike to rich and poor points out the only true riches to which all men both can and should aspire. Finally, the priest is the most valorous leader in that crusade of expiation and penance to which We have invited all men of good will. For there is need of reparation for the blasphemies, wickedness and crimes which dishonor humanity today, an age perhaps unparalleled in its need for the mercy and pardon of God. The enemies of the Church themselves well know the vital importance of the priesthood; for against the priesthood in particular, as We have already had to lament in the case of Our dear Mexico, they direct the point of their attacks. It is the priesthood they desire to be rid of; that they may clear the way for that destruction of the Church, which has been so often attempted yet never achieved.
Source: Ad Catholici Sacerdotii (Vatican.va)