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.