Jamieson, Fausset and Brown on Hebrews 12:28

28. receiving--as we do, in prospect and sure hope, also in the possession of the Spirit the first-fruits. This is our privilege as Christians.

let us have grace--"let us have thankfulness" [Alford after Chrysostom]. But (1) this translation is according to classical Greek, not Paul's phraseology for "to be thankful." (2) "To God" would have been in that case added. (3) "Whereby we may serve God," suits the English Version "grace" (that is Gospel grace, the work of the Spirit, producing faith exhibited in serving God), but does not suit "thankfulness."

acceptably--Greek, "well-pleasingly."

reverence and godly fear--The oldest manuscripts read, "reverent caution and fear." Reverent caution (same Greek as in He 5:7; see on He 5:7) lest we should offend God, who is of purer eyes than to behold iniquity. Fear lest we should bring destruction on ourselves.