So often, when you read a commentary on part of the Bible you're studying, you have pages and pages of material but the commentator doesn't seem to be puzzling over the same details of the passage as you are.
How refreshing when the commentator asks exactly the questions you were asking, and has some very sensible things to say.
Before I read his notes, I was puzzling over three questions in particular. It turns out that, once you've seen the link between Psalm 2 and this passage, all becomes clear.
Why the emphasis on what the apostles heard at the Transfiguration?
The words on the mountain come from Psalm 2:7, coupled with Isaiah 42:1. In the context of this part of 2 Peter, Jesus' transfiguration is when Jesus is vested with God's authority as his chosen king, and specifically the one will be God's agent for judgement. The brightness and splendour that they saw, as eyewitnesses, is important. However the words they heard are especially clear.
“The Transfiguration is the basis for the Parousia expectation because it is God’s appointment of Jesus to a role which he has not yet exercised but will exercise at his coming in glory. This view of the significance of the Transfiguration in 2 Peter also accounts for the great emphasis which these verses place on the divine voice, to the exclusion of other aspects of the Transfiguration.” (Page 220)
Why is it called a holy mountain?
It can’t be because the site of the transfiguration was venerated, because there wasn’t any tradition as to its location until much later. Neither can it be a link to Sinai, because Sinai was only ever called “holy” when God was there, and it’s never called the “holy mountain” elsewhere in the OT. So it must be a reference to Zion in general and Psalm 2:6 in particular.
“Of course, there is no intention of locating the Transfiguration historically on Mount Zion; the point is to identify the event with the prophecy in Psalm 2 to explain its theological significance. The apostle, claims 2 Peter, were there with Jesus when God appointed him his king, and they themselves heard the divine decree.” (221)
2 Peter 1:16 sees the Transfiguration as evidence that the apostolic teaching about the second coming was not made up. How does the Transfiguration establish credibility to the second coming? How does it follow logically from Jesus being transfigured into brightness on a mountain that he'll return on a future day to judge the world?
Once again, it's all about Psalm 2. The apostles were eyewitnesses to the Transfiguration. Peter's readers believed the historicity of the Transfiguration, so he starts with what they know to be true and shows them just how reliable this is. They saw it, and (as I argued above) they heard it.
“It is unlikely that the writer expected his readers to question the historicity of the Transfiguration. They would no doubt already know, in oral or written form, a tradition of the Transfiguration account and would accept it with the rest of the Gospel traditions as resting on the eyewitness testimony of the apostles. What the author of 2 Peter does is to point out to them that this tradition forms a basis for the expectation of the Parousia. His argument is that one reason why the apostles were so confident in their teaching about the Parousia is that (as the readers know and accept) they witnessed the Transfiguration. Since the readers accept this eyewitness testimony to the Transfiguration as reliable, they can also rely on the teaching which the apostles based on it: the prophecy of the Parousia.” (Pages 216-7)
This means they were present when God the Father spoke the decree we read about in Psalm 2, which is where Jesus is installed as the one who will judge the nations. So they know that the Psalm 2 king is installed on Zion, the holy hill, because they were there when it happened. They therefore know that the judgement Psalm 2 speaks of will surely come.
Not everything about Bauckham's commentary is brilliant, and it's not to be trusted on every point made. Specifically, he thinks Peter didn't write 2 Peter, and at times this working assumption contorts his exegesis considerably. Nevertheless, read critically, he has some absolutely brilliant insights.