2 Peter 1:16

16 For we did not follow cleverly devised tales when we made known to you the power and coming of our Lord Jesus Christ, but we were eyewitnesses of His majesty.

Peter here is straight forward about some of the “things” he (and the other Apostles) had been teaching them (v. 12)—mainly about Jesus’ second coming. He wants them to know for sure that he and the other Apostles were not telling them made-up stories or myths (as some false teachers probably were accusing them of doing). So, Peter was confirming to them that they were physically with Jesus when He was telling them of how He would one day come to earth in great power to set up His kingdom.