The Catechism of Trent explains to us the Church's Doctrine on why it is the right and obligation of all the faithful, in union with Priests, as part of the "internal priesthood" to offer up spiritual sacrifices of all they do, in union with the Sacrifice of the Mass throughout the world, which is the Sacrifice of Christ. Our Lady of Fatima also spoke of offering up one's daily duties. "Twofold Priesthood
But as Sacred Scripture describes a twofold priesthood, one internal and the other external, it will be necessary to have a distinct idea of each to enable pastors to explain the nature of the priesthood now under discussion.
The Internal Priesthood
Regarding the internal priesthood, all the faithful are said to be priests, once they have been washed in the saving waters of Baptism. Especially is this name given to the just who have the Spirit of God, and who, by the help of divine grace, have been made living members of the great High-Priest, Jesus Christ; for, enlightened by faith which is inflamed by charity, they offer up spiritual sacrifices to God on the altar of their hearts. Among such sacrifices must be reckoned every good and virtuous action done for the glory of God.
Hence we read in the Apocalypse: Christ hath washed us from our sins. in his own blood and hath made us a kingdom, and priests to God and his Father. In like manner was it said by the Prince of the Apostles: Be you also as living stones built up, a spiritual house, a holy priesthood offering up spiritual sacrifices acceptable to God by Jesus Christ; while the Apostle exhorts us to present our bodies a living sacrifice holy , pleasing unto God your reasonable service. And long before this David had said: A sacrifice to God is an afflicted spirit: a contrite and humble heart O God thou wilt not despise. All this clearly regards the internal priesthood.
The External Priesthood
The External Priesthood, on the contrary, does not pertain to the faithful at large, but only to certain men who have been ordained and consecrated to God by the lawful imposition of hands and by the solemn ceremonies of holy Church, and who are thereby devoted to a particular sacred ministry."
Thus, the importance of "offering up" (together with the Sacrifice of Christ, and with all the Holy Masses offered up by Priests in the world) all our actions for the glory of God, including good works and our daily cross. Offering everything up in union with the Body and Blood of the Lord to expiate the daily sins committed in the world is very pleasing to God and obtains great graces for the Church and for us all.