The self-testimony of Mark about his gospel: In the narration of the Transfiguration
(Mark 9, 2-13) Jesus takes the disciples Peter, John and James with him and leads them up a high mountain. Jesus is transfigured in an apparition and stands between the apparitions of Moses and Elijah who talk to him.
Peter would like to make three shelters (= three temples, three churches), one for Moses, one for Elijah, one for Jesus. But the vision ends, the disciples are alone with Jesus.
Jesus stands here in a row with the mythic heroes of Judaism, Moses and Elijah: he himself becomes a mythic hero. Mark also made a statement about his gospel. The Pentateuch records the story of Moses; the books of Kings and of prophets records the story of Elijah and the prophets.
In his gospel Mark records about the story of Jesus with the same authority as the scriptures of Judaism. Mark can only gain the authority this requires if he writes his gospel according to an agreement with the leaders of the apostle churches.
Therefore the founding fathers of each of the three churches were named witnesses of the major theological creeds. Mark writes right from the start for the unified Christian church and right from the start he claims canonical importance.