In the first reading Paul told the people of Corinth that he was a true Apostle, and he told them that they should not accept any new Gospel from anyone who was not an Apostle.
The longest living of the Apostles was St. John, and before he died, he named a man named Polycarp to follow him as bishop of Smyna, and he entrusted the Gospel to him. In time, Polycarp entrusted the Gospel to Iraenius, making him the bishop to follow him.
In the next century when strangers began preaching their own gospel, Irenaeus explained to them that Jesus had entrusted his Gospel to the Apostles and to the bishops they appointed to follow them.
It is the same down to our time. In 1962 Pope John XXIII invited the world’s twenty-six hundred bishops to come together for the Second Vatican Council. There, praying each day for light from the Holy Spirit, in a democratic fashion, over four years those successors of the Apostles established a present-day expression of the Gospel of Christ.