In Scripture “to fear God” is to be in awe of his power and knowledge. To fear God requires a daring heart!
Only a heart that fears God can be joyful. Fear is a word that we typically interpret as referring to a state of emotional distress in the face of some danger to our personal safety. The term “fear of the Lord” appears over 100 times in the Old Testament. For example: And now, Israel, what does the Lord, your God ask of you but to fear the Lord, your God, and follow his ways exactly, to love and serve the Lord your God with all your heart and all your soul (Deuteronomy 10:12). However, in the New Testament, the term is only mentioned two times and has been transformed into a sense of awe that is joyful rather than horrified. It is the gift of fear that gives us an unmistakable and irrefutable sense of God’s closeness and his ultimate victory over all evil in the world.
The gift of fear of the Lord gives us a greater sense of the greatness of God that should spark in our hearts a sense of amazement and awe that could bring us down to our knees. If we abandon astonishment we are left with a mediocre piousness.
To St. Bonaventure fear of the Lord was "the most beautiful tree planted in the heart of a holy man which God waters continuously" [II.6]. This “most beautiful tree” bears the precious fruit of love and reverence for God. Fear of the Lord for St. Bonaventure was the sort of trembling before experiences of God’s majesty that we hear perfectly encapsulated in the hymn:
Let all mortal flesh keep silence,
And with fear and trembling stand;
Ponder nothing earthly minded,
For with blessing in His hand,
Christ our God to earth descendeth
Our full homage to demand.
The fear that St. Bonaventure had in mind is sort of a continuum that spans a certain range—depending upon one's perfection in the life of grace—from "servile fear" to "filial fear" to a fear cast out by love which has taken over one’s whole heart (cf. 1 John 4:17-18).