Because Christ himself gave the apostles the authority: “Whose sins you forgive are forgiven them” (John 20:23), and the Catechism attests: “Confession of all grave sins … is imposed on the penitent by divine law” (CCC 1456). Thomas Aquinas explains: “No one but the priest is the minister of the sacrament of Penance … the minister cannot apply a suitable remedy if he does not know the sin, and he obtains this knowledge through confession” (Summa Theologiae, Suppl. q.8, a.1; q.6, a.1).
This means that Thomas Aquinas does not understand the sacrament of Penance as merely a private dialogue between God and the individual, but as a real remedy instituted by Christ himself, which he administers through the ordained minister—the priest.
When Aquinas says: “No one but the priest is the minister of the sacrament of Penance”, he means: Only the priest has received from Christ the sacramental authority to validly forgive sins. This authority is part of the “power of the keys” that Jesus entrusted to the apostles in John 20:23.
When he adds: “The minister cannot apply a suitable remedy if he does not know the sin”, he uses a medical image. Just as a doctor cannot prescribe effective medicine without knowing the illness, so the priest as a spiritual physician cannot impose an appropriate penance or grant sacramental absolution without knowing the nature of the guilt.
And when he says: “He obtains this knowledge through confession”, this is the reason for oral confession: the confession makes the sin manifest in the spiritual tribunal, so that the priest can act in the name of Christ. For Aquinas, therefore, personal confession to the priest is not a human addition, but a necessary component of the sacrament of Penance instituted by God.
Bible:
John 20:21–23
“Jesus said to them again, ‘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. Whose sins you forgive are forgiven them, and whose sins you retain are retained.’”
James 5:16
“Therefore, confess your sins to one another and pray for one another, that you may be healed.”
Leviticus 5:5–6 (Prefiguration in the Old Covenant)
“When someone becomes guilty in any of these matters, he must confess how he has sinned and bring as his guilt offering … to the priest; the priest will make atonement for him for his sin.”
2 Corinthians 5:18–20
“All this is from God, who reconciled us to himself through Christ and gave us the ministry of reconciliation. … So we are ambassadors for Christ, God making his appeal through us.”
Thomas Aquinas
(Summa Theologiae, Supplementum, q.8, a.1):
“The grace given in the sacraments flows from the head to the members. Therefore, only one who serves the true body of Christ is a minister of the sacraments … This belongs only to the priest, who is able to consecrate the Eucharist. Now, since grace is given in the sacrament of Penance, no one but the priest is the minister of this sacrament; consequently, sacramental confession … is to be made only to a priest.”
(Supplementum, q.6, a.1):
“… by confessing his sin, a person submits himself to a minister of the Church, so that through the sacrament of Penance administered by him he may obtain forgiveness; but the minister cannot apply a suitable remedy if he does not know the sin, and he obtains this knowledge through the confession of the penitent.”
Catechism of the Catholic Church:
CCC 1456:
“Confession of all mortal sins of which one is aware after a careful examination of conscience is required of the penitent by divine law. The Church therefore requires all the faithful to confess all mortal sins not yet confessed, of which they are aware, by kind and number, in the sacrament of Penance.”
CCC 1461:
“Since Christ entrusted to his apostles the ministry of reconciliation, bishops who are their successors, and priests, the bishops’ collaborators, continue to exercise this ministry. … Priests continue the mission of reconciliation that the Lord entrusted to his apostles and their successors.”