Apostolic succession is the doctrine that the authority of the apostles has been transmitted through an unbroken chain of ordained bishops from the apostles to the present. Irenaeus of Lyon (~180 AD) develops it as the primary argument against Gnosticism: the Gnostics claim secret traditions from the apostles, but we can list the bishops of Rome, Antioch, and Alexandria in unbroken succession back to the apostles — and they teach nothing like Gnosticism. Apostolic succession serves as the guarantee of doctrinal continuity and the source of sacramental validity in Catholic, Orthodox, and Anglican ecclesiology. The bishop who ordains validly must himself have been validly ordained in this succession. The Reformation rejected this doctrine (Luther’s doctrine of the priesthood of all believers) or modified it (Anglicanism claims to have retained apostolic succession through its episcopal hierarchy). Catholic and Orthodox churches do not recognize Protestant ordinations as valid.