One of the items that was seen as significant at Vatican Council II was the priesthood of the laity, the notion that we are all priests in the order of Melchizedek. In the Catholic Church, there is a clear distinction made between those who have been given priestly faculties by the successors of the Apostles through the sacrament of Holy Orders (the clergy) and those who have not (the laity). Typically, the former group are referred to as priests because they preside in persona Christi at the Holy Sacrifice of the Mass.
The role of a priest has always been inherently intertwined with sacrifice in the communities of the descendants of Abraham; within the Judeo-Christian tradition, the priest facilitates the sacrificial offering to God that reconciles the person to God in unity with the community. So in what sense could we Christians who are incorporated into the Body of Christ by baptism be acting as a priest?
It is clear that we do not stand at the altar and ask for the action of the Holy Spirit to transform the bread and wine into the most precious Body and Blood of our Lord. It is clear that we do not even have the faculties to do so if we have not received the indelible mark from the sacrament of Holy Orders.
Nonetheless, as we grow in holiness through the process of sanctification, we do gradually come to exist more and more in our daily lives as a priest does in persona Christi during the liturgy. The fundamental vocation of the Catholic clergyman is the same as the fundamental vocation of the lay Catholic; we are called to become Christlike, to "put on Christ" as Paul wrote in the Epistles.
How do we become more like Christ? Christ lived his life for us, humbling Himself to become one of us so that we might see God truly through our experience of a human being. Christ offered his suffering and death up for us as a sacrifice so that we as a human community could have the opportunity to be reconciled to God. Christ opened up the possibility of eternal life in unity with the divine family of God by his resurrection, accomplishing a transformation of the entirety of human existence so that it can participate in the divine life of love.
In the same way, we are called to humble ourselves (and especially priests as Pope Francis has reiterated), living our lives for the good of others to help them see God in Heaven more clearly by our lowly existence on Earth. And in the same way, we are called to offer our sufferings and death up to God as part of our participation in the reconciliation of ourselves to God. Likewise, we are called to resurrection as a new person, transformed in becoming like Christ to participate in the divine life of love for eternity, showing God by our earthly lives that this is what we truly seek to become with all the power of our bodies, wills, intellects, hearts, and souls.
We are called to see God through our experience of human beings, to let the imago dei provide a lens through which we can begin to glimpse the goodness of Deus. Likewise, we are called to empty ourselves of the darkness of the ego that obscures His light in us, allowing others to see God through their experience of us, directing their gaze through the anthropos to the brilliance of the Theos.
We are called to offer up our sufferings and death for the love of our fellow human beings so that they might be reconciled to God. Our suffering for others becomes a bridge by which they can understand Christ's suffering for them, a constant example of Christ's sacrifice by which He offered us the opportunity to be reconciled to God, a reminder of both the price and the beauty of the sacrifice on the cross.
We are called to transform our entire existence so that we can participate in the divine life of love, showing by our lives of sacrifice that it is possible to be reconciled to God and to the human community, pointing to the God who came to us as a living sacrifice by existing in the process of sanctification as an increasingly clear and bright image of a living sacrifice for the sake of love.
Thus we operate in persona Christi, priests in the Body of Christ that is made of His Church, a body of persons whose members are always sacrificing for one another their egotistical desires, suffering with one another while serving one another as an offering to God so that they can be reconciled to God, always pointing to God and eternal life with Him in Heaven by their earthly lives of sacrificial service.
In our acts of loving service to others and to God, we love to death our egotistical striving after anything less than the divine life of love God offers us, acting our way into a transformation of our entire existence to leave behind the version of our selves that do not want to find joy in that divine life of love and become the version of our selves that do seek to fully participate in that divine life of love.