“Early” Christianity ended with the Council of Nicaea which set the canon of Scripture. That was in the 325, partway through the fourth century.
Assume that by the fifth century a priesthood had divided the Church from The Laity. Blame many things but of course human ego and turf conflict got involved.
John Wycliffe (died of old age, 1384 or the end of the fourteenth century) produced a Bible in the English of his day (Chaucer was about a hundred years prior and we call his language Middle English.) When the Church found his grave they burned the bones and cast the ashes onto the waters. Jan Hus died at the stake for his effort to bring the Bible to the Masses, in their own language, in 1486 – the end of the fifteenth Century.
Martin Luther completed the Bible in German fifty-ish years later, and due to the cash politics of the day (Northern Germany decided not to let Rome drain their coffers to build St. Peter’s etc.) the nobility guarded Luther. But “the Church” i.e. the Roman Catholic Church held onto its right to “interpret” Scripture for several hundred more years.
Corrupt priests occur in all faiths and all ages. Corrupted teaching is another topic, and naturally occurs more when the Word is known less.