Sometimes it can be easy to forget that Jesus is fully present at each Mass and that the priest or bishop is only an instrument for his miraculous power.
The priest can not bring about anything without the supernatural help of God working through him.
In the person of Christ
The Catechism of the Catholic Church explains that Jesus is the primary agent of the Eucharist:
Christians come together in one place for the Eucharistic assembly. At its head is Christ himself, the principal agent of the Eucharist. He is high priest of the New Covenant; it is he himself who presides invisibly over every Eucharistic celebration. It is in representing him that the bishop or priest acting in the person of Christ the head (in persona Christi capitis) presides over the assembly, speaks after the readings, receives the offerings, and says the Eucharistic Prayer.
St. John Paul II highlights this phrase in his encyclical Ecclesia de Eucharistia and recalls its significance:
The expression repeatedly employed by the Second Vatican Council, according to which "the ministerial priest, acting in the person of Christ, brings about the Eucharistic Sacrifice," was already firmly rooted in papal teaching. As I have pointed out on other occasions, the phrase in persona Christi "means more than offering 'in the name of' or 'in the place of' Christ.' In persona means in specific sacramental identification with the eternal High Priest who is the author and principal subject of this sacrifice of his, a sacrifice in which, in truth, nobody can take his place."
This teaching is one of the reasons why the priest says in the first person, "This is my body."
The priest is not referring to his own body, but to Jesus' body, speaking the words as if Jesus were speaking them.
In fact, Jesus is speaking those words through the priest in a mystical way, becoming present in the bread and wine through the words of the priest.
The priest is simply God's instrument. While he may visibly appear to be the main celebrant of the Mass, it is Christ himself who is the one working behind the scenes through every action of the priest.
