In a nutshell, the teachings of Christ were given to the Apostles. These men appointed bishops to succeed them and to them they entrusted the protection of the Christ’s teachings. The appointing of bishops down to our day is known as Apostolic Succession and the passing on of the Christ’s teachings is called Apostolic Tradition. While other Christian writers had referred to both in the decades prior to St. Irenaeus, Against Heresies provides us with the role of the Pope as the source of unity in the Church and protector of the Faith.
Keeping in mind that this was written around the year 180 AD, check out the words of St. Irenaeus:
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 [to demonstrate] the succession of these men to our own times; those who neither taught nor knew of anything like what these [heretics] 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 lameless 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 [to the Church], but if they should fall away, the direst calamity.
Since, however, it would be very tedious, in such a volume as this, to reckon up the successions of all the Churches, we do put to confusion all those who, in whatever manner, whether by an evil self-pleasing, by vainglory, or by blindness and perverse opinion, assemble in unauthorized meetings; [we do this, I say,] by indicating that tradition derived from the apostles, of the very great, the very ancient, and universally known Church founded and organized at Rome by the two most glorious apostles, Peter and Paul; as also [by pointing out] the faith preached to men, which comes down to our time by means of the successions of the bishops. For it is a matter of necessity that every Church should agree with this Church [in Rome], on account of its preeminent authority, that is, the faithful everywhere, inasmuch as the apostolic tradition has been preserved continuously by those [faithful men] who exist everywhere.
The blessed apostles, then, having founded and built up the Church, committed into the hands of Linus the office of the episcopate [as second Pope]… [Irenaeus then lists the popes up until his own day] Eleutherius does now [reign as Pope], in the twelfth place from the apostles, hold the inheritance of the episcopate. In this order, and by this succession, the ecclesiastical tradition from the apostles, and the preaching of the truth, have come down to us. And this is most abundant proof that there is one and the same vivifying faith, which has been preserved in the Church from the apostles until now, and handed down in truth.