‘For we did not follow cunningly devised fables (myths), when we made known to you the power and coming (parousia) of our Lord Jesus Christ, but we were eyewitnesses of his majesty.'
He wants them to know that unlike his opponents he had not been inventing cunningly devised myths (stories conveying religious truth in acted out worship) when he had told them about the power and coming of our Lord Jesus Christ, because he had actually already had a foretaste of it, and had actually seen His glory. He and his fellow-disciples had indeed been eyewitnesses of His majesty. This had demonstrated His real power and glory, as it had been revealed in the cross and resurrection, and was now being revealed in the spreading and effectiveness of the Gospel, and it was proof that His future coming in glory and excellence would be a genuine coming with a real presence. For they had had a foretaste of it here on earth. This was no heavenly myth. There had been nothing fictitious about it. It had been a genuine experience. The Lord (sovereign over all things) Jesus (God made man and Saviour) Christ (the One Anointed to fulfil God's saving purposes) was here.
Thus the false teachers by their false ideas were ‘denying the Lord Who bought then', that is, they were denying that He as ‘the Lord' had really lived among men and had died for them and had risen again. They were denying His presence and power. But their denial was contradicted by the fact that the disciples had visibly seen His power and glory and Parousia while He was on earth, and they had seen it even as His death was being discussed, a sight which had emphasised His power and coming as God made man. The manifested One was also the crucified One.
Compare how Mark 9:1 emphasises that the Transfiguration was linked with ‘the Kingly Rule of God coming in power'. The false teachers were denying both His power and His Parousia (presence, coming). For this denial see 2 Peter 3:3-4. But the Transfiguration had been a foretaste and manifestation of both. For there His glory had been physically manifested and revealed as real, and because of that they could be sure that it would be real too at His final Parousia, which would simply make manifest what was already true, that Christ reigns over the earth. He then goes on to emphasise this by giving an independent account of the Transfiguration.
In other words as a result of His coming and presence Christ has never ceased to be present on earth. His Parousia (according to Peter) includes the totality of His work as the One Who has come from the Father, which is continually manifested in power and will in the end be manifested in glory. His presence and coming was revealed, partially invisibly, while He was here living on earth, and openly in glory at the Transfiguration; it is invisibly true as He goes forward with His people in their present responsibility to disciple all nations (Matthew 28:20; Mark 9:1); and it will be openly manifested in the end in His final glorious appearing. Both aspects of His Parousia manifest openly in glory what has been genuinely so from the time when the Father sent the Son to be the Saviour of the world (1 John 4:14), for in a essence they are one Parousia.
The danger is that we see ‘the body of Christ' on earth as something other than Christ, as though Christ is in Heaven and His body is on the earth. But that is not so (1 Corinthians 12:12). The ‘body of Christ' is the risen, powerful, invisible Christ Himself present on earth as one with His people. We are His body as united with Him (Romans 6:5; Romans 6:11). We are His body because we have been made one with Him. The idea is never used in any other way. See 1 Corinthians 10:16-17; 1 Corinthians 12:12-13; Ephesians 1:19 to Ephesians 2:6; Ephesians 2:13 with 16, 20; 2 Peter 3:17; Colossians 2:9-10; Colossians 2:19. Thus Christ is here present on earth in us.
This is the glorious paradox of the Gospel, that while Christ in His manifested manhood is absent from us, in His Godhood He is present within us, and living out His life through us. And what will happen at His final Parousia will be that He will be manifested in His glorified manhood in the glory which He had with the Father before the world was (John 17:5).
And Peter wants his readers to know that this is not a myth. It is not simply something which is part of an acted out ceremony. For this idea of ‘myths' compare 2 Peter 2:1; 2 Peter 2:3 where the false teachers bring in destructive heresies denying the reality of the cross (‘denying the Lord Who bought them'), and use feigned words. They claimed to be Christian teachers, but their reality was simply a Hellenistic ritual (and possibly a drug induced state) built up around the name of Christ, and probably connected with other saviours.