Apostolic succession is the claim that the authority Christ gave the apostles has been transmitted in an unbroken line to bishops through the laying on of hands. It is asserted by the Catholic, Eastern Orthodox, Oriental Orthodox and Assyrian churches, by the Anglican Communion, by the Old Catholic churches and by some Lutheran bodies, and it governs their understanding of ordination, of the validity of sacraments, and of what makes a church a church.
The early evidence is real but thinner than the developed doctrine. Clement of Rome, writing about 96, describes the apostles appointing successors and providing for the succession to continue. Irenaeus, arguing in the 180s against Gnostic teachers who claimed a secret tradition, replies that the churches founded by apostles can produce public lists of their bishops, and that traceable succession is therefore the guarantee of authentic teaching, his argument is about doctrine, and succession is the evidence for it. Cyprian in the third century ties succession to the unity of the episcopate. Historians generally hold that the single bishop over a city emerged gradually, and that some first- and second-century communities were led by a college of presbyters, so that the later doctrine reads back a clarity the earliest period did not have.
Four disputes follow. The first is whether succession is of persons or of doctrine: most of the Reformers argued that the true apostolic succession is fidelity to apostolic teaching, and that a chain of hands guarantees nothing if what is taught has changed. The second is whose orders are valid. Leo XIII's bull Apostolicae Curae (1896) declared Anglican ordinations "absolutely null and utterly void" on grounds of defect of form and intention in the Edwardine ordinal; the Archbishops of Canterbury and York replied in Saepius Officio (1897) rejecting both the reasoning and the historical account, and the question remains open. Orthodox churches judge by their own criteria and have not spoken with a single voice. The third is whether succession belongs to the essence of the church or to its good order, the difference between saying a church without it is no church and saying it is a church lacking something valuable. The fourth is ecumenical: the Faith and Order text Baptism, Eucharist and Ministry (1982) and later bilateral dialogues have sought language in which episcopal succession is a sign of the church's apostolicity without being its sole guarantee, an approach that has enabled several agreements of communion and that critics on both wings regard as evading the question.