Objection 1. It would seem that God cannot be feared. For the object of fear is a future evil, as stated above (I-II, 41, 2,3). But God is free of all evil, since He is goodness itself. Therefore God cannot be feared.
Objection 2. Further, fear is opposed to hope. Now we hope in God. Therefore we cannot fear Him at the same time.
Objection 3. Further, as the Philosopher states (Rhet. ii, 5), "we fear those things whence evil comes to us." But evil comes to us, not from God, but from ourselves, according to Hosea 13:9: "Destruction is thy own, O Israel: thy help is... in Me." Therefore God is not to be feared.
On the contrary, It is written (Jeremiah 10:7): "Who shall not fear Thee, O King of nations?" and (Malachi 1:6): "If I be a master, where is My fear?"
I answer that, Just as hope has two objects, one of which is the future good itself, that one expects to obtain, while the other is someone's help, through whom one expects to obtain what one hopes for, so, too, fear may have two objects, one of which is the very evil which a man shrinks from, while the other is that from which the evil may come. Accordingly, in the first way God, Who is goodness itself, cannot be an object of fear; but He can be an object of fear in the second way, in so far as there may come to us some evil either from Him or in relation to Him.
From Him there comes the evil of punishment, but this is evil not absolutely but relatively, and, absolutely speaking, is a good. Because, since a thing is said to be good through being ordered to an end, while evil implies lack of this order, that which excludes the order to the last end is altogether evil, and such is the evil of fault. On the other hand the evil of punishment is indeed an evil, in so far as it is the privation of some particular good, yet absolutely speaking, it is a good, in so far as it is ordained to the last end.
In relation to God the evil of fault can come to us, if we be separated from Him: and in this way God can and ought to be feared.
Reply to Objection 1. This objection considers the object of fear as being the evil which a man shuns.
Reply to Objection 2. In God, we may consider both His justice, in respect of which He punishes those who sin, and His mercy, in respect of which He sets us free: in us the consideration of His justice gives rise to fear, but the consideration of His mercy gives rise to hope, so that, accordingly, God is the object of both hope and fear, but under different aspects.
Reply to Objection 3. The evil of fault is not from God as its author but from us, in for far as we forsake God: while the evil of punishment is from God as its author, in so far as it has character of a good, since it is something just, through being inflicted on us justly; although originally this is due to the demerit of sin: thus it is written (Wisdom 1:13-16): "God made not death... but the wicked with works and words have called it to them."
Source: Summa Theologica (New Advent)