It is within the power of all, therefore, in every Church, who may wish to see the truth, to contemplate clearly the tradition of the apostles manifested throughout the whole world; and we are in a position to reckon up those who were by the apostles instituted bishops in the Churches, and the succession of these men to our own times; those who neither taught nor knew of anything like what these rave about. For if the apostles had known hidden mysteries, which they were in the habit of imparting to “the perfect” apart and privily from the rest, they would have delivered them especially to those to whom they were also committing the Churches themselves. For they were desirous that these men should be very perfect and blameless in all things, whom also they were leaving behind as their successors, delivering up their own place of government to these men; which men, if they discharged their functions honestly, would be a great boon , but if they should fall away, the direst calamity. (Ad. Hear. III.I.I)
The Gnostics (those who claimed a wide range of doctrines, abilities, traditions, and developments which were their own inventions) would say that the Apostles had secret mysteries or perhaps did not fully understand everything – yet Irenaeus defended against that, because he stood in succession from those same Apostles who had in turn given to the overseers what they knew who had given to their successors what the Apostles had known.