In 2 Peter 1:16 what does Peter mean by “eyewitnesses of his majesty”?
Peter along with James and John were eyewitnesses to the transfiguration , in fulfillment of the prophetic word that was told to them six days earlier, that some of his followers would " not taste death until they see the kingdom of God after He has come with power.”( Mark 9:1) Thus Peter was able to inform his readers the power and presence of our Lord Jesus Christ.
2 Peter 1:19 (NASB)
19" So we have the prophetic word made more sure, to which you do well
to pay attention as to a lamp shining in a dark place, until the day
dawns and the morning star arises in your hearts."
The Majesty of His Transfiguration.
Mark 9:1-7 NASB
1 And Jesus was saying to them, “Truly I say to you, there are some of
those who are standing here who will not taste death until they see
the kingdom of God after it has come with power.”
2 "Six days later, Jesus *took with Him Peter and [a]James and John,
and *brought them up on a high mountain by themselves. And He was
transfigured before them; 3 and His garments became radiant and
exceedingly white, as no launderer on earth can whiten them. 4 Elijah
appeared to them along with Moses; and they were talking with Jesus."
5 "Peter *said to Jesus, “Rabbi, it is good for us to be here; let us
make three [b]tabernacles, one for You, and one for Moses, and one for
Elijah.” 6 For he did not know what to answer; for they became
terrified. 7 Then a cloud [c]formed, overshadowing them, and a voice
[d]came out of the cloud, “This is My beloved Son, [e]listen to Him!”