Fear of the Lord has “Lord” as genitive, but are we meant to understand an objective genitive, where “the Lord” is the object of fear; or a genitive of description, like “a man of great passions” or “object of desire” where the genitive describes or characterizes its complement?
Thomas says that if the genitive is objective and simpliciter that the fear in question is evil, since it would amount to saying that God is an evil that one should flee. There is also an objective genitive secundum quid, namely the fear of the Lord under the qualification that one is a sinner deserving divine punishment. Even though divine justice is not an object of fear simpliciter, it can be under the qualification that one has sinned. This second sense is what Christ clearly uses in Matthew 10:28. That said, fear of the Lord in this sense is not perfect virtue.
Fear of the Lord is only a virtue if one takes “Lord” not as an objective genitive but as a descriptive one, namely, fear of the Lord is a fear into which “The Lord” enters descriptively, neither as an object feared simpliciter nor secundum quid, but as an object of devotion giving rise to a fear of things opposed to God.
Just Thomism 