Christ is the center of our confession of faith in the gospel, both in creed and public testimony. Surely, the readers would not give up Christ, whom they had professed, and reject what He had done for them, if they could understand the superiority of His person and work.
Hold fast does not refer to how to be saved or remain saved. It means rather that perseverance in faithfulness is proof of real faith.
Commentary from the MacArthur study Bible, notes for Hebrews 3:1,6.
Therefore, since all these things will be dissolved,
what manner of persons ought you to be,
in holy conduct and godliness,
2 Peter 3:11
The “elements” are the atomic components into which matter is ultimately divisible, which make up the composition of all the created matter. Peter means that the atoms, neutrons, protons, and electrons are all going to disintegrate (v. 11). The whole of the physical, natural earth in its present form, with its entire universe will be consumed. As man from atoms creates destructive bombs that burn a path of death, so God can disintegrate the whole universe in an explosion of atomic energy. He exercises sovereign control over every person, object, and event, and not one molecule in the universe is outside that dominion.
Commentary from the MacArthur study Bible, notes for 2 Peter 3:7,10, and Revelation 1:8.