“the fear of the Lord is clean, enduring forever; the rules of the Lord are true, and righteous altogether.” -Psalm 19:9
The expression the fear of the Lord refers to the way the Lord’s subjects (believers) serve him. Cleanness is chasteness. The fear of the Lord is not a distressing fear but one that endures forever because it is chaste in and of itself. The fear of the Lord does not require of us that which is unclean, unrighteous, unsure, or imperfect. Further, it cultivates in us an awe and respect for the Lord. That is, it cultivates the chaste love of the bride for her bridegroom.
Finally, the Lord’s verdicts (favorable or unfavorable), his formal decrees are always pronounced judiciously because in him there is no variation or shadow of turning in his righteousness.
Thus, knowing the Lord’s judgments are always righteous and true, the fear of the Lord (our service to our God and King) is clean and endures forever.
As Augustine of Hippo notes on this passage, “but that chaste fear wherewith the Church, the more ardently she loves her Spouse, the more carefully does she take heed of offending Him, and therefore, perfect love casteth not out this fear (1 John 4:18), but it endureth for ever.”