But, beloved, &c.— This is exactly like St. Paul's way of closing and softening any thing that he had said which sounded terrible and dreadful. See for examples, 2 Thessalonians 2:13.Ephesians 4:20. Romans 8:9.
Related Commentaries of Hebrews 6:9
Hebrews 6:9
9 But, beloved, we are persuaded better things of you, and things that accompany salvation, though we thus speak.