Adam Clarke on Exodus 30:21

And it shall be a statute for ever - To continue, in its literal meaning, as long as the Jewish economy lasted, and, in its spiritual meaning, to the end of time. What an important lesson does this teach the ministers of the Gospel of Christ! Each time they minister in public, whether in dispensing the Word or the Sacraments, they should take heed that they have a fresh application of the grace and spirit of Christ, to do away past transgressions or unfaithfulness, and to enable them to minister with the greater effect, as being in the Divine favor, and consequently entitled to expect all the necessary assistances of the Divine unction, to make their ministrations spirit and life to the people. See Clarke's note on Exo 29:20.