"For we did not follow cunningly devised fables when we made known to you the power and coming of our Lord Jesus Christ, but were eyewitnesses of His majesty."
The word “for” connects the thought with Peter’s previous words. The sense is, “I want you to always be able to call these things to mind after I’m gone, because they are true. We didn’t make up clever stories. We were eyewitnesses of what we are handing off to you.”
So the foundation of our faith is the apostolic witness to Jesus Christ. We can note four things about this apostolic witness: The apostles explicitly deny making up tales about Jesus Christ; The apostolic witness centers exclusively on the person of Jesus Christ as the glorious, majestic Son of God, equal with the Father; The apostolic witness affirmed that Jesus Christ is coming again in power and glory; The apostles affirm being eyewitnesses of the majesty of Jesus Christ on the mount of transfiguration.