From: Romans 12:1-2
Solidarity in the Mystical Body
 I appeal to you therefore, brethren, by the mercies of God, to present your bodies as a living sacrifice, holy and acceptable to God, which is your spiritual worship. Do not be conformed to this world but be transformed by the renewal of your mind, that you may prove what is the will of God, what is good and acceptable and perfect.
1. In the New Testament Christians are clearly called to offer sacrifices to God-- no longer sacrifices of animals, as in the Old Law, but offerings of themselves. This new kind of worship must take a spiritual form, as Jesus told the Samaritan woman, rather than a purely material form: it must be something living, holy, not merely external and formal, and pleasing to God (cf. Jn 4:23). "It is by the apostolic preaching of the Gospel that the people of God is called together and gathered so that all who belong to this people, sanctified as they are by the Holy Spirit, may offer themselves 'a living sacrifice, holy and acceptable to God' (Rom 12:1)" (Vatican II, "Presbyterorum Ordinis", 2).
The basis of this priestly meaning of Christian life is to be found in the sacrament which makes us members of Christ's body: "Through Baptism all of us have been made priests of our lives, 'to offer spiritual sacrifices acceptable to God through Jesus Christ' (1 Pet 2:5). Everything we do can be an expression of our obedience to God's will and so perpetuate the mission of the God-man" (St. J. Escriva, "Christ Is Passing By", 96).
Every day the Christian can and should offer himself along with Christ in the Holy Mass: "If the oblation whereby the faithful in this Sacrifice offer the divine victim to the heavenly Father is to produce its full effect [...] they must also offer themselves as victim, desiring intensely to make themselves as like as possible to Jesus Christ who suffered so much, and offering themselves as a spiritual victim with and through the High Priest himself" (Pius XII, "Mediator Dei", 25). From this it follows that the whole Christian life and the struggle which it implies are imbued with deep priestly significance: "If I renounce everything I possess, if I carry the cross and follow Christ, I have offered a holocaust on the altar of God, or if I burn up my body in the fire of charity [...] I have offered a holocaust on the altar of God [...]; if I mortify my body and abstain from all concupiscence, if the world is crucified unto me and not me unto the world, then I have offered a holocaust on the altar of God and I am become a priest of my own sacrifice" (Origen, "In Lev. Hom.", 9, 9).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.