Jesus had 12 eyewitnesses of His glory and those were the twelve disciples, they lived many incredible moments with the Master, moments that today we read them and we thought how good it would be to have had such privilege. The twelve disciples could be part of great miracles performed by Jesus, of great mercies, of words with a broadly celestial wisdom, they simply had the greatest privilege, but this was not everything, since many other people could witness these things, but they were simply there for the miracle, since at the time of Jesus being delivered, they were screaming: Release Barabbas and crucify Jesus!
It is not doubt that these disciples, later converted into the apostles of the early church, had to suffer much ignorance and lack of faith from other people to proclaim the name of Christ, and that is why Peter writes in his second letter:
16 But he was rebuked for his wrongdoing by a donkey—an animal without speech—who spoke with a human voice and restrained the prophet’s madness.
17 These people are springs without water and mists driven by a storm. Blackest darkness is reserved for them. 18 For they mouth empty, boastful words and, by appealing to the lustful desires of the flesh, they entice people who are just escaping from those who live in error.
2 Peter 2:16-18
In the first verse Peter is practically saying that he had not taught the gospel by emotion or by any fashion of the moment, or as if it were a fairy tale. He is telling them that the things he was teaching them he has see seen them, he had witnessed it at certain moments. Let us remember that moment when Peter was on a mountain and Jesus was transfigured. Peter was able to witness that glory and now he is talking about it and he does not do so out of emotion, he does it as an eyewitness, as he who saw with his own eyes multiple miracles done by Jesus.