By studying it
One of the reasons I converted from Protestantism (non-denominational, Pentecostal, Lutheran)
Read the Early Church Fathers. Read The Didache. It takes a lot of mental gymnastics to read early accounts of how the Church was and what they believed and to still say that it showed that The Early Church was not Catholic. All claims that The Catholic Church was not the early Church (The Mormon church, Jehova’s witnesses or even primitive Baptists claim the early church believed as they believe) can never be backed up by any evidence from The early writings or recorded history. The only other thing to say is that these early writings are incorrect and suddenly some new denomination got everything right. I trust those that actually learned from The Apostles themselves and were around in the times of The Early Church.
Exactly. I’m a former Protestant and, after studying the writings of the Early Church,I had to conclude the Early Church was Catholic.
I also noticed that most of the sects and religions that came later always looked back to the Catholic Church as the evil institution that destroyed everything, with God needing to send some reformer or prophet to fix everything up.
For Protestants God had to send Luther, Calvin, Zwingli, (or even King Henry the VIII) to restore the Gospel that the Catholic Church had lost. For Muslims God had to send Muhammad to restore the original Gospel that the Catholic Church had supposedly destroyed. For Baha’i God had to send the Bab and Bahá’u’lláh to fix what the Catholic Church destroyed and restore Jesus’ original teachings. For Jehovah’s Witnesses God had to send Charles Russell to restore the original Gospel that the Catholic Church lost. For Seventh Day Adventists, God had to send Ellen G White to restore the true gospel after the Great Whore of Babylon, the Catholic Church, destroyed it. And lastly, for Mormons, God had to send Joseph Smith to restore what the Catholic Church had destroyed.
(Heck, even atheists and secularists often believe that Pagan Rome was a wonderful time of liberal-minded ‘goodness,’ that the Catholic Church destroyed when it became dominant in Europe, and the Enlightenment restored this liberal-minded secularist greatness.)
All of them have one thing in common-- that God couldn’t keep the Catholic Church from destroying Jesus original teachings after a few centuries, and so God had to send someone else many centuries later to fix it up.
They all also have one other thing in common-- they all focus on Scripture (or even write new Scriptures) to prove their points that Jesus teachings were corrupted, yet none of them started by reading the Early Church Fathers’ to prove there really ever was doctrinal corruption by the Catholic Church that occurred at some point after Christ.
That’s why Bl Cardinal Newman said “to be deep in history is to cease to be Protestant.” The study of history also ceases Mormonism, Islam, Baha’i, etc.