In persona Christi
In persona Christi is a Latin phrase meaning “in the person of Christ.” It is an important concept in many Christian traditions in varying degrees. The extended phrase, In persona Christi capitis, “in the person of Christ the head,” was introduced in the Catechism of the Catholic Church (CCC paragraphs 1548, 1581).
In Roman Catholicism, and Orthodoxy, the priest is ordained to act in the person of Christ, most significantly in pronouncing the key words of a sacramental rite; for example, the Words of Institution, in the Eucharist, through which the consecrated bread becomes, by the power of Jesus Christ the High Priest, the Body of Christ and the wine becomes His Precious Blood; also in the normal form of Baptism, and the Anointing of the Sick; in the anointing of the Holy Spirit in Confirmation; and in the ordination of a deacon, of a priest, and the consecration of a bishop. The sacramental grace of Holy Matrimony is normally witnessed by a deacon, priest or bishop in persona Christi, but actually initiated and given by an already baptized Christian lay couple "in Christ", a man and a woman, to each other in licit marriage, by irrevocable consent, and throughout their lives.
The priest and bishop act in the person of Christ the head in their leadership of the Church through the power of Christ the Lord conferred on them by means of the sacrament of Holy Orders.