What is the Justification of a priest acting in persona Christi?

In the Person of Christ

In the Catholic Church, the priest serving at the altar is considered to be acting in persona Christi Capitis, that is, "in the person of Christ, the Head [of the Church]." By this, the Church means that the priest and the bishop, when they carry out their priestly functions, do so as Christ's representatives (Benedict XVI, Munus docendi, General Audience, 14 September 2010). The belief that the priest acts in persona Christi Capitis when carrying out the sacraments is old. There are dozens of excellent teachings from the Popes on this subject.

Pope Benedict XVI was careful to point out that priests do not represent Christ as if he was absent, for Christ is always present in the Church, but rather, the priest represents Christ as Christ represented the Father (Munus docendi, paragraphs 2, 3). Consider that Jesus "... is the reflection of God’s glory and the exact imprint of God’s very being..." Hebrews 1:3a. (All Bible passages are taken from the Revised Standard Version, Catholic Edition, of the Bible.) Also, "He is the image of the invisible God..." Colossians 1:15. Jesus, speaking about his teaching, said, "My teaching is not my own but his who sent me" (John 7:16). Moreover, in his High Priestly prayer, Jesus stated to the Father, with respect to his apostles who were soon to face severe trials upon his arrest and execution:

Now they know that everything you have given me is from you; for the words that you gave to me I have given to them, and they have received them and know in truth that I came from you; and they have believed that you sent me. John 17:7-8.

Speaking to his apostles, Jesus said, "Whoever listens to you listens to me, and whoever rejects you rejects me, and whoever rejects me rejects the one who sent me" Luke 10:16. Because Jesus is truly present in and with his apostles as they teach and make disciples, wherever they go (Matthew 28:16-20), any act for or against the apostle is an act for or against Jesus. (In a way, Matthew 28:16-20 applies to all believers, which is why Jesus could say to Saul on the Damascus Road in Acts 9:4, "Why are you persecuting me?" Nevertheless, Jesus did not give all believers the authority that he gave the apostles in Luke 10:16, nor are all believers called to the ministerial priesthood.)

Furthermore, Jesus gave his apostles his own authority over sin when He said: "Peace be with you. As the Father has sent me, so I send you." When he had said this, he breathed on them and said to them, “Receive the Holy Spirit. If you forgive the sins of any, they are forgiven them; if you retain the sins of any, they are retained." John 20:21-23.

We can see this reflected in St. Paul's second letter to the Church at Corinth, when he wrote, "Anyone whom you forgive, I also forgive. What I have forgiven, if I have forgiven anything, has been for your sake in the presence of Christ" (2 Corinthians 2:10). The phrase "in the presence" uses the Greek word prosōpon (pros=towards; ops=an eye, literally, towards the eye or face), which can be translated as appearance, presence, or person. Modern English translations of this passage mostly use "in the presence of Christ," but many older English translations used "in the person of Christ" (for example, the King James and Douay-Rheims translations). The idea St. Paul gives is that Christ is truly present in the act of forgiving the sinner. That is, Christ forgave the sinner through Paul for the sake of the sinner and for the sake of the Corinthian Church.

Finally, returning to the High Priestly prayer, Jesus told the Father that he had imparted the glory of the Father to all believers, and he prayed for perfect unity of all believers with each other and with the Father and the Son, as the Father and the Son unite perfectly in each other.

... that they all [all believers in Christ] may be one. As you, Father, are in me and I am in you, may they also be in us,so that the world may believe that you have sent me. The glory that you have given me I have given them, so that they may be one, as we are one, I in them and you in me, that they may become completely one, so that the world may know that you have sent me and have loved them even as you have loved me. John 17:21-23.

Thus, returning to Pope Benedict XVI, "The Lord makes his own action present in the person who carries out these gestures [the priestly consecration of bread and wine at Eucharist, absolution for sins]" Munus docendi, paragraph 4. In other words, the priest is not the ambassador of a distant king, but being united fully with Christ in his office as priest, Christ is in the priest, and the work of the priest is the work of Christ, not symbolically, but in reality, because of the mystical union of the priest at the altar with Jesus Christ, our Great High Priest.

In the Catechism of the Catholic Church (CCC), the specific paragraphs you should read are 1548 – 1551; however, the context for those four paragraphs is in paragraphs 1539 – 1553, and you might want to read all of them (about 4.5 printed pages in my copy of the Catechism).

1548 In the ecclesial service of the ordained minister, it is Christ himself who is present to his Church as Head of his Body, Shepherd of his flock, high priest of the redemptive sacrifice, Teacher of Truth. This is what the Church means by saying that the priest, by virtue of the sacrament of Holy Orders, acts in persona Christi Capitis:

"It is the same priest, Christ Jesus, whose sacred person his minister truly represents. Now the minister, by reason of the sacerdotal consecration which he has received, is truly made like to the high priest and possesses the authority to act in the power and place of the person of Christ himself (virtute ac persona ipsius Christi)."

"Christ is the source of all priesthood: the priest of the old law was a figure of Christ, and the priest of the new law acts in the person of Christ."

CCC, paragraph 1548.

Paragraph 1548 cites Lumen Gentium, paragraphs 10 and 28; Sacrosanctum concilium, paragraph 33; Christus Dominus, paragraph 11; and Presbyterorum ordinis, paragraphs 2 and 6, as authority for that statement. Moreover, the first block quoted passage in paragraph 1548 comes from Acta Apostolicae Sedis, volume 39 (1947), page 548, which cites St. Thomas Aquinas, Summa Theologica, III, 22, 4c. The second block quoted passage in paragraph 1548, cites the same section from Summa Theologica. Furthermore, almost identical language is used by Pope Pius XII in his encyclical Mediator Dei, which is referred to with respect to the quotes from Summa Theologica.

In CCC 1549, the Catechism explains that priests and bishops are the visible sign of the presence of Christ as head of the church "... in the midst of the community of believers", citing Lumen Gentium, paragraph 21. In this paragraph, the Catechism also refers to St. Ignatius of Antioch's statement that the bishop is "the living image of God the Father" (citations omitted).

As paragraph 1550 of CCC makes clear, the human minister is not preserved from all sin, error, or human weakness. Rather, in all of the sacraments, the Holy Spirit guarantees "... that even the minister's sin cannot impede the fruit of grace..." In his other acts, the minister is not guaranteed by the Holy Spirit to be free of human weakness and may "... leave[] human traces that are not always signs of fidelity to the Gospel and consequently can harm the apostolic fruitfulness of the Church" (CCC 1550).

As I stated, there are numerous teachings on this subject by various popes and doctors of the Church. Much of the material is available on the Vatican website in English. Various Catholic apologetics sites also have bits and pieces of this material. I pulled from all of them in assembling this answer. I hope it helps answer your question adequately.

In Christ, Hreodbeorht