He declares that he was an eyewitness to the majesty of Jesus Christ. There is more to the word ‘eyewitness’ than the way we commonly use the word.
Here is what William Barclay wrote about it.
“In the Greek usage of Peter’s day this was a technical word. We have already spoken about the Mystery Religions. These Mystery Religions were all of the nature of passion plays, in which the story of a god who lived, suffered, died, and rose again, never to die again, was played out. It was only after a long course of instruction and preparation that the worshipper was finally allowed to be present at the passion play, and to be offered the experience of becoming one with the dying and rising God. When he reached the stage of being allowed to attend the actual passion play, he was an initiate, and the technical word to describe him was in fact epoptes; he was a prepared and privileged eyewitness of the experiences of God.” (“The Letters of James and Peter”, rev. ed. [Philadelphia: Westminster, 1976], 367)