Irenaeus

Irenaeus (~140 - 203 AD) was Bishop of Lugdunum, now Lyons, France.  He was born in Smyrna, raised in a Christian family, and heard Polycarp's teaching in his younger days. It is unclear if he was martyred or not.

Irenaeus wrote a number of books, but only two complete works survive.  The most important is Against Heresies, a five-book series refuting Gnosticism.  The second is A Demonstration of Apostolic Preaching, which may have been used to instruct new believers.

Gnosticism gets its name from the Greek word gnosis, meaning knowledge.  There were many different schools and variations of it, but they all taught some special kind of knowledge was required for salvation.  They freely borrowed from various religions, including Judaism, Roman and Greek traditions, Zoroastrianism, and eventually Christianity.  Because it included Christian concepts and terminology, increasing numbers of Christians were adopting these gnostic beliefs and Irenaeus argued strongly to show how they were contrary to scripture and the apostles' teaching.

In the process of refuting Gnosticism, Irenaeus helped establish what we now call the New Testament.  Because heretical writings started surfacing falsely claiming to be written by one of the apostles, Irenaeus cited those writings that were authentic.  He referred to both the Old Testament and the apostles' writings as "Scripture."

Irenaeus introduced some new concepts, including apostolic succession and the authority of church tradition.  Because the heretics claimed to know more than the apostles did, Irenaeus not only pointed to the apostles' writings, but also to the fact that some of the apostles appointed the first church elders.  Implied in that is that there was first-hand instruction, which ensured clear inheritance of doctrine.  Since heretics were introducing new ideas, church traditions were seen as another way of ensuring consistent doctrine.
But again, when we refer them to that tradition which originates from the apostles, [and] which is preserved by means of the succession of presbyters in the churches, they object to tradition, saying that they themselves are wiser not merely than the presbyters, but even than the apostles, because they have discovered the unadulterated truth. ... It comes to this, therefore, that these men do now consent neither to Scripture nor to tradition.  Against Heresies, Book III, ch. 2.

By documenting a clear succession of appointments of church leaders, later generations would use that as a justification for the authority of the pope.  Pointing to church tradition was fine for the doctrinal points that the church got right, but as the church accepted unbiblical practices, tradition became a justification for ignoring scripture and elevating the decisions of church leaders above all else.