It recently came up in discussion with a friend “whether church leaders can be valid if they are not in apostolic succession.” I offer these two Church Fathers in support of the idea that both doctrinal unity and apostolic succession are necessary for the validity and trustworthiness of a church leader. (But I observe that, if the Church teaches the importance of apostolic succession, then it is impossible to be in doctrinal unity with the Church and deny it.)
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Irenaeus
Against Heresies [A.D. 189]
“[I]t 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 certain gift 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 schismatics 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” (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” (4:33:8).
Also, a bonus identifying the Church in Rome as primary:
“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” (3:3:2).
Tertullian
Demurrer Against the Heretics [A.D. 200]
“[W]hat it was which Christ revealed to them [the apostles] can, as I must here likewise prescribe, properly be proved in no other way than by those very churches which the apostles founded in person, by declaring the gospel to them directly themselves . . . If then these things are so, it is in the same degree manifest that all doctrine which agrees with the apostolic churches—those molds and original sources of the faith must be reckoned for truth, as undoubtedly containing that which the churches received from the apostles, the apostles from Christ, [and] Christ from God. Whereas all doctrine must be prejudged as false which savors of contrariety to the truth of the churches and apostles of Christ and God. It remains, then, that we demonstrate whether this doctrine of ours, of which we have now given the rule, has its origin in the tradition of the apostles, and whether all other doctrines do not ipso facto proceed from falsehood” (21).
“But if there be any [heresies] which are bold enough to plant [their origin] in the midst of the apostolic age, that they may thereby seem to have been handed down by the apostles, because they existed in the time of the apostles, we can say: Let them produce the original records of their churches; let them unfold the roll of their bishops, running down in due succession from the beginning in such a manner that [their first] bishop shall be able to show for his ordainer and predecessor some one of the apostles or of apostolic men—a man, moreover, who continued steadfast with the apostles. For this is the manner in which the apostolic churches transmit their registers: as the church of Smyrna, which records that Polycarp was placed therein by John; as also the church of Rome, which makes Clement to have been ordained in like manner by Peter” (32).