The fear of the Lord is the foundation and summit of the other gifts of the Holy Spirit. It can properly be understood in two ways, the second building on the first: awe and wonder, and filial fear.
When one begins to get a sense of the glory of God, it inspires reverential awe. To be aware of your own nothingness, your complete and utter dependence on the One Who Is, the Creator. We can become fearful in the sense that we have just had the ground taken out from under our existential selves; we realize just how truly fragile we are. We also can become fearful in the sense that we suddenly find ourselves in the presence of a great mystery, the Mystery of mysteries. We start to get a smidgeon of an idea of how little we truly know in every possible way that there is to know. We also begin to see that the wisdom of God is more important and more imperative to seek after than knowledge and wisdom of the world. This is why it says in the Scripture that “fear of the Lord is the beginning of wisdom” (Prov 9:10) and not just the beginning, but also “a school of wisdom” (Prov 15:33). It fosters humility which is necessary for all other virtues and gifts of the Holy Spirit.
However, one must move beyond this type of the fear of the Lord. To progress in the spiritual life it is necessary for one to enter more deeply into the practice of this virtue so that it is no longer merely awe and wonder, but is now filial fear. Not only is it not possible to progress in the spiritual life if one does not progress in the fear of the Lord, but remaining at the state of awe and wonder can actually have adverse effects on one’s soul and move them further from God. One can come to emphasize the mystery of all things to the point that it leads to doctrinal and spiritual indifference. This can also eventually lead to a false fear of God which can manifest itself in a fear of God’s vengeance (“Catholic” guilt is an example of this) or even a form of nihilism. But the Holy Scriptures tells us: “Better to have little and with it fear of the Lord than to have treasure and with it anxiety.” (Prov 15:16) One who possesses fear of the Lord in its fullest sense is free from anxiety, no matter the circumstances of their life at any given moment. St. John of the Cross explains it thus:
Thus when the soul attains to the perfect possession of the spirit of fear, she has the spirit of love insofar as that fear, which is the last of the seven gifts, is filial. And perfect filial fear arises from perfect paternal love. So when the divine Scripture wishes to point out that a person is perfect in charity, it says such a one is God-fearing. (St. John of the Cross, The Spiritual Canticle,stanza 26, para. 3)
Fear of the Lord is also love of the Lord; it is a filial fear arising from a paternal love. It has moved beyond awe and has become a manifestation of mystical union. In the first (awe) a personal relationship is not necessarily implied and if there is one (as is common) it is the first steps of the relationship entailing openness and seeking progression in the relationship. In the second (filial) there is necessarily a relationship, a relationship founded on love. The love of the most Holy Trinity for the soul and the love of the soul for God. It is a fear on the part of the beloved not to harm the Lover. A husband should be scared of hurting his wife. Not because of how she may retaliate, but because he loves her so much that he only desires her happiness and to do something that causes her pain also causes him pain. This is the fullest sense of the fear of the Lord. A fear of not loving Him; not because of twisted conceptions of wrath and retribution, but for fear of hurting the One we love above all else, the One who has become our All.