2 Do you see what advantage has come of fear? If fear were not a good thing, fathers would not have set tutors over their children; nor lawgivers magistrates for cities. What can be more grievous than hell? Yet nothing is more profitable than the fear of it; for the fear of hell will bring us the crown of the kingdom. Where fear is, there is no envy; where fear is, the love of money does not disturb; where fear is, wrath is quenched, evil concupiscence is repressed, and every unreasonable passion is exterminated.
And even as in a house, where there is always a soldier under arms, no robber, nor house-breaker, nor any such evil doer will dare to make his appearance; so also while fear holds possession of our minds, none of the base passions will readily attack us, but all fly off and are banished, being driven away in every direction by the despotic power of fear. And not only this advantage do we gain from fear, but also another which is far greater. For not only, indeed, does it expel our evil passions, but it also introduces every kind of virtue with great facility.
Where fear exists, there is zeal in almsgiving, and intensity of prayer, and tears warm and frequent, and groans fraught with compunction. For nothing so swallows up sin, and makes virtue to increase and flourish, as a perpetual state of dread. Therefore it is impossible for him who does not live in fear to act aright; as, on the other hand, it is impossible that the man who lives in fear can go wrong.
Source: Homilies on the Statues (New Advent)