Today we honor Irenaeus who was raised in Smyrna by Polycarp, who followed John the Apostle. Irenaeus, as a boy, over and over listened to Polycarp repeating the teachings of Jesus that he had heard from John.
Then, in the Second Century, some so-called Christians began filling Rome with new “Christian teachings,” they claimed they were receiving directly from angels. With that, the Pope asked Irenaeus to come to Rome to straighten those people out.
After meeting with all those innovators, Irenaeus kindly brought them around to seeing that the teachings of Jesus were completed before his Ascension when he entrusted his Gospel to the Apostles.
Irenaeus wrote that since the death of the Apostles, the true teachings of Jesus could be learned by consulting the bishops appointed by the Apostles. That holds today.
In 1870, the First Vatican Council declared the pope to be infallible, but following on that, the members of Rome’s Curia came to see themselves, as infallible when they spoke in the pope’s name. The error of that came to light when the Curia began subjecting the world’s bishops to their decrees.
Pope John XXIII straightened us out on that by inviting all the world’s Catholic bishops to attend the Second Vatican Council, where they as one clarified what had been the one Gospel of Christ.