These protests against the Catholic Church has been rehearsed multiple times in its history. Particularly by the Gnostics in the 2nd century, who claimed to have true knowledge of the true gospel, that even the apostles themselves corrupted the gospel taught by Christ. Living in the 1st century, without a Bible in hand, how can one know which of opposing versions of the gospel is the Truth? The early Church responded to the challenge by the Gnostics by appealing to apostolic succession. St. Irenaeus, who lived in the 2nd century, explains how the Apostolic Tradition was to be found, to whom it was entrusted, and how it was preserved. He said,
"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 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 [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, on account of its preeminent authority, that is, the faithful everywhere, inasmuch as the tradition has been preserved continuously by those [faithful men] who exist everywhere."
That is why 2 Thess 2:15 exhorts us to stand firm and hold to the traditions which you were taught, whether by word of mouth or by letter from us.
These apostolic teachings, or the Oral Tradition, has been passed down to us, through the unbroken apostolic succession for 2000 years (even the task of preserving the original manuscripts of the Bible you are holding) down to the present 266th successor, in the name of Francis.
John17