Adam Clarke on Exodus 20:20

And Moses said - Fear not: for God is come to prove you, and that his fear may be before your faces - The maxim contained in this verse is, Fear not, that he may fear - do not fear with such a fear as brings consternation into the soul, and produces nothing but terror and confusion; but fear with that fear which reverence and filial affection inspire, that ye sin not - that, through the love and reverence ye feel to your Maker and Sovereign, ye may abstain from every appearance of evil, lest you should forfeit that love which is to you better than life. He who fears in the first sense can neither love nor obey; he who fears not in the latter sense is sure to fall under the first temptation that may occur. Blessed is the man who thus feareth always.