Vicar of Christ (from Latin Vicarius Christi) is a term used in different ways and with different theological connotations throughout history. The original notion of a vicar is as an "earthly representative of Christ", but it's also used in the sense of "person acting as parish priest in place of a real person." The title is now used in Catholicism to refer to the bishops and more specifically was historically used to the Bishop of Rome (the pope).
During the history of Christianity, the title of Vicar of Christ was used in different ways, with implications for theological, pastoral or different time.
An early appearance of a similar concept of the Vicar of Christ is mentioned in the Epistle to the Magnesians of St. Ignatius, Bishop of Antioch, who was possibly a disciple of both John the Apostle and Saint Peter, with a pastoral sense, written between the years AD 88 and 107 "your bishop presides in the place of God (...)". Although Ignatius did not explicitly use the term Vicar of Christ, he sets out the concept, with regard to local bishops. More recently, the Second Vatican Council's Dogmatic Constitution on the Church Lumen gentium noted that bishops are "vicars and ambassadors of Christ," and the Catechism of the Catholic Church notes that each bishop governs his diocese "[a]s Christ's vicar."
The first recorded use of the term "Vicar of Christ" is found in the epistles of Tertullian in the late 2nd and early 3rd centuries, referring to the Holy Spirit, that is, as Christ is not physically performing miracles in the Church, Holy Spirit acts as his Vicar on his behalf, performing miracles and preventing the Church from error. Other roles Tertullian attributed to the Holy Spirit as Vicar were: the direction of discipline, the revelation of the Scriptures, the reformation of the intellect, and the advancement toward the
better things. It is unknown whether this term was more widely used in the early Church.
The third use of the term Vicar of Christ appears in the 5th century, in a synod of bishops to refer to Pope Gelasius I. The theological connotations of the title got a pastoral sense, evoking the words of Christ to the Apostle Peter, regarded by the first Catholic Pope in John 21:16-17, "Feed my lambs... Feed my sheep", so Christ made Peter his vicar and pastor with the responsibility to feed his flock (i.e. the Church) in his own place.
However, the use of the title to refer to the popes in the early Church was unstable, and several variants of the use of Vicar were used for the Pope, as "Vicar of Peter", indicating that they were the successors of St. Peter, "Vicar of the Prince of the Apostles" or "Vicar of the Apostolic See", among other variants. This title is used by the Roman Missal in their prayers for a dead pope, and the oath of allegiance to St. Boniface to Pope Gregory II. Since 1200, Popes have consistently used this title, although Pope Francis recently moved this title to a different section of the 2021 Annuario Pontificio. Insisting that he — and he alone—had the right to remove bishops from office, Pope Innocent III appealed to the title of Vicar of Christ. Occasionally, Popes like Nicholas III used "Vicar of God" as an equivalent title The 2012 edition of the Annuario Pontificio gives "Vicar of Jesus Christ" as the second official title of the Pope (the first being "Bishop of Rome").
Another use of the title, with a different meaning, appeared in the Eastern Churches, in use between the century fifth and sixth, the term was used to refer to the Byzantine emperor, showing the apex of caesaropapism. Though decisions on doctrine, liturgy and spirituality were left to the bishops (most notably in Church Councils, where the Emperor often played a key role since the time of Constantine the Great), the Emperor constantly had tremendous influence on the Church, which was increasingly charged with tasks in the service of the crown, such as supervising temporal authorities.