Apostolic succession is the line of bishops stretching back to the apostles. All over the world, all Catholic bishops not just of the Roman Catholic Church are part of a lineage that goes back to the time of the apostles, something that is impossible in Protestant denominations (most of which do not even claim to have bishops). The role of apostolic succession in preserving true doctrine is illustrated in the Bible. To make sure that the apostles' teachings would be passed down after the deaths of the apostles, Paul told Timothy, "What you have heard from me before many witnesses entrust to faithful men who will be able to teach others also" (2 Tim. 2:2). In this passage he refers to the first three generations of apostolic succession-his own generation, Timothy's generation, and the generation Timothy will teach.
The Church Fathers, who were links in that chain of succession, regularly appealed to apostolic succession as a test for whether Catholics or heretics had correct doctrine. This was necessary because heretics simply put their own interpretations, even bizarre ones, on Scripture. Clearly, something other than Scripture had to be used as an ultimate test of doctrine in these cases.
Thus the early Church historian J. N. D. Kelly, a Protestant, writes, "Where in practice was the apostolic testimony or tradition to be found? . . . The most obvious answer was that the apostles had committed it orally to the Church, where it had been handed down from generation to generation. . . . Unlike the alleged secret tradition of the Gnostics, it was entirely public and open, having been entrusted by the apostles to their successors, and by these in turn to those who followed them, and was visible in the Church for all who cared to look for it" (Early Christian Doctrines, 37). For the early Fathers, "the identity of the oral tradition with the original revelation is guaranteed by the unbroken succession of bishops in the great sees going back lineally to the apostles. . . . An additional safeguard is supplied by the Holy Spirit, for the message committed was to the Church, and the Church is the home of the Spirit. Indeed, the Church's bishops are . . . Spirit-endowed men who have been vouchsafed 'an infallible charism of truth'" (ibid.). Thus on the basis of experience the Fathers could be "profoundly convinced of the futility of arguing with heretics merely on the basis of Scripture. The skill and success with which they twisted its plain meaning made it impossible to reach any decisive conclusion in that field" (ibid., 41).
Pope Clement I. "Through countryside and city the apostles preached, and they appointed their earliest converts, testing them by the Spirit, to be the bishops and deacons of future believers. Nor was this a novelty, for bishops and deacons had been written about a long time earlier. . . . Our apostles knew through our Lord Jesus Christ that there would be strife for the office of bishop. For this reason, therefore, having received perfect foreknowledge, they appointed those who have already been mentioned and afterwards added the further provision that, if they should die, other approved men should succeed to their ministry" (Letter to the Corinthians 42:4-5, 44:1-3. A.D. 80).
Hegesippus. "When I had come to Rome, I visited Anicetus, whose deacon was Eleutherus. And after Anicetus died, Soter succeeded, and after him Eleutherus. In each succession and in each city there is a continuance of that which is proclaimed by the law, the prophets, and the Lord" (Memoirs, cited in Eusebius, Ecclesiastical History 4:22 .A.D. 180).
Irenaeus. "It is possible, then, for everyone in every church, who may wish to know the truth, to contemplate the tradition of the apostles which has been made known to us throughout the whole world. And we are in a position to enumerate those who were instituted bishops by the apostles and their successors down to our own times, men who neither knew nor taught anything like what these heretics rave about" (Against Heresies 3:3:1.A.D. 189). "But since it would be too long to enumerate in such a volume as this the successions of all the churches, we shall confound all those who, in whatever manner, whether through self-satisfaction or vainglory, or through blindness and wicked opinion, assemble other than where it is proper, by pointing out here the successions of the bishops of the greatest and most ancient church known to all, founded and organized at Rome by the two most glorious apostles, Peter and Paul-that church which has the tradition and the faith with which comes down to us after having been announced to men by the apostles. For with this Church, because of its superior origin, all churches must agree, that is, all the faithful in the whole world. And it is in her that the faithful everywhere have maintained the apostolic tradition" (ibid., 3:3:2).
"Polycarp also was not only instructed by apostles, and conversed with many who had seen Christ, but was also, by apostles in Asia, appointed bishop of the church in Smyrna, whom I also saw in my early youth, for he tarried on earth a very long time, and, when a very old man, gloriously and most nobly suffering martyrdom, departed this life, having always taught the things which he had learned from the apostles, and which the Church has handed down, and which alone are true. To these things all the Asiatic churches testify, as do also those men who have succeeded Polycarp down to the present time" (ibid., 3:3:4)."Since therefore we have such proofs, it is not necessary to seek the truth among others which it is easy to obtain from the Church; since the apostles, like a rich man depositing his money in a bank, lodged in her hands most copiously all things pertaining to the truth, so that every man, whosoever will, can draw from her the water of life. . . . For how stands the case? Suppose there arise a dispute relative to some important question among us, should we not have recourse to the most ancient churches with which the apostles held constant conversation, and learn from them what is certain and clear in regard to the present question?" (ibid., 3:4:1)."It is incumbent to obey the presbyters who are in the Church-those who, as I have shown, possess the succession from the apostles; those who, together with the succession of the episcopate, have received the infallible charism of truth, according to the good pleasure of the Father. But it is also incumbent to hold in suspicion others who depart from the primitive succession, and assemble themselves together in any place whatsoever, either as heretics of perverse minds, or as schismatic’s puffed up and self-pleasing, or again as hypocrites, acting thus for the sake of lucre and vainglory. For all these have fallen from the truth" (ibid., 4:26:2). "The true knowledge is the doctrine of the apostles, and the ancient organization of the Church throughout the whole world, and the manifestation of the body of Christ according to the succession of bishops, by which succession the bishops have handed down the Church which is found everywhere" (ibid., 4:33:8).