For we have not followed cunningly devised fables, when we made known unto you the power and coming of our Lord Jesus Christ, but were eyewitnesses of his majesty. For he received from God the Father honour and glory, when there came such a voice to him from the excellent glory, This is my beloved Son, in whom I am well pleased. And this voice which came from heaven we heard, when we were with him in the holy mount. (2 Peter 1:16-18 KJV)
Peter, having been with Jesus during His earthly ministry, appeals to the fact that he was an eyewitness of the events of His life on earth.
I have been on jury duty in the past and am presently. In a court of law much trust is put into eye witness accounts of the case at hand. Peter was an eyewitness to Jesus' earthly ministry. We should put much trust in what he has to say. Yet many refuse to believe anything the Bible has to say...even an eye witness account!
Many have trouble with the idea that there is three persons but one God. It is rather mind blowing. But here Peter talks about his experience with Jesus on the Mt of transfiguration. Jesus, God the Son, was on the Mt and God the Father spoke saying, "This is my beloved Son, in whom I am well pleased."
It was not two Gods but one. Jesus was not talking to Himself. It was the Father speaking. Some get tripped up on the Triune nature of God, Father, Son and Holy Spirit. Because they don't understand how it works they dismiss it as a fable. It's interesting that these same people can not explain or understand how the Internet works, or how electricity works. Yet, they believe it works and use it. Why can't they afford God the same consideration?
The real issue isn't that they think they should be able to understand how the triune nature of God "works", but rather, they expect God to conform to their own image or idea of who or what He is, and if He doesn't they reject what the Bible teaches about Him.
I can take great comfort in knowing that as an eyewitness, Peter knows where of he speaks and if he says the things about Jesus were not cunningly devised fables, then it seems he has authority to make that claim. If he says God the Father spoke and Jesus (God the Son) was present, we can believe him. He was there!