It is through Holy Orders that “the mission entrusted by Christ to his apostles continues to be exercised in the Church until the end of time: thus it is the sacrament of apostolic ministry.”
[Catechism of the Catholic Church 1536]
Catholics believe that the one true priest is Jesus Christ, but that during His earthly life, Jesus gave a share of His priesthood to the Apostles. Through the laying on of hands, the Apostles in turn ordained more men to serve as priest of Jesus Christ (2 Tim 1:6). For over 2000 years, through apostolic succession, the priesthood has been handed on in the Church.
The priesthood is not about power, but on the contrary, it is a profound call to serve (Matthew 20:25-28, John 13:2-20). Priests carry on the ministry of Jesus through the sacraments and preaching the Word - they are called to serve and strengthen the people of God, so they may truly be the light of the world.
From Jesus’ conception until his resurrection he was filled with the Holy Spirit, thus established by God the Father as our high priest and remains our high priest. Ordination to the priesthood is always a call and gift from God. Those who seek priesthood respond generously to God’s call using the words of the prophet, “Here I am, send me” (Is. 6:8). This call from God can be recognized and understood from the daily signs that disclose his will to those in charge of discerning the vocation of the candidate. – United States Catholic Catechism for Adults