The “fear of the Lord,” as it is usually used in Catholic language, is one of the seven gifts of the Holy Spirit, which come from St Jerome’s Latin version of the Book of Isaiah. In Isaiah 11, the principle spiritual gifts of the coming Messiah are prophesied. These gifts are poured into the Christian soul at baptism and increased and restored throughout life by the reception of the holy sacraments and growth in the life of grace.
The gift of the fear of the Lord perfects our hope of remaining in God’s grace and someday being with him in the happiness of heaven. This means that this fear is a fear caused by love. We fear to lose God by sin, since we love him, and we have a reverent fear and awe of him because he is so beautiful and great and powerful. This fear is called “filial” fear, because it like the fear of a child who is afraid to hurt the feelings of his father because of their mutual love. It is different from what is called “servile” fear, which is based on punishment, like the fear of a slave for his master.
The fear of the Lord is source of great peace and happiness; it is thus very different from worldly fears of suffering or punishment and rather it helps us endure pain because of our love for the Savior.