The Coptic church claims “St. Mark” started their church? I have never heard this before.
So, the “secret knowledge” they have comes from....?
It is not secret knowledge. According to Eusebius of Caesarea in “Ecclesiastical History” written in the early 4th century Mark traveled to Alexandria and became the first Bishop of Alexandria. The Copts claim to be the descendents of that first Christian Church in Egypt.
You can know that Mark founded the church in Alexandria the same way you can know that Homer wrote the Iliad or Cyrus was the king of Persia: by using accepted historiographical rules of evidence: letters, chronicles, inscriptions, monuments,lists of people commemorated in public celebrations, proclamations of a legal nature, etc.
The writings of people like Eusebius, the inscriptions on the walls of the catacombs, the names listed in the anaphora --- these would be good examples.
BTW, the whole idea that Mark WROTE the "Gospel according to St Mark" comes from this same capital-T Tradition. You won't find the authorship of any of the for Gospels attested to in the Bible itself.
Permit me to excuse myself now from this argument. I need to go back to sleep.
Please pray for me, a sinner.