Objection 1. It seems that the will of expression is not to be distinguished in God. For as the will of God is the cause of things, so is His wisdom. But no expressions are assigned to the divine wisdom. Therefore no expressions ought to be assigned to the divine will.
Objection 2. Further, every expression that is not in agreement with the mind of him who expresses himself, is false. If therefore the expressions assigned to the divine will are not in agreement with that will, they are false. But if they do agree, they are superfluous. No expressions therefore must be assigned to the divine will.
On the contrary, The will of God is one, since it is the very essence of God. Yet sometimes it is spoken of as many, as in the words of Psalm 110:2: "Great are the works of the Lord, sought out according to all His wills." Therefore sometimes the sign must be taken for the will.
I answer that, Some things are said of God in their strict sense; others by metaphor, as appears from what has been said before (13, 3). When certain human passions are predicated of the Godhead metaphorically, this is done because of a likeness in the effect. Hence a thing that is in us a sign of some passion, is signified metaphorically in God under the name of that passion. Thus with us it is usual for an angry man to punish, so that punishment becomes an expression of anger. Therefore punishment itself is signified by the word anger, when anger is attributed to God. In the same way, what is usually with us an expression of will, is sometimes metaphorically called will in God; just as when anyone lays down a precept, it is a sign that he wishes that precept obeyed. Hence a divine precept is sometimes called by metaphor the will of God, as in the words: "Thy will be done on earth, as it is in heaven" (Matthew 6:10). There is, however, this difference between will and anger, that anger is never attributed to God properly, since in its primary meaning it includes passion; whereas will is attributed to Him properly. Therefore in God there are distinguished will in its proper sense, and will as attributed to Him by metaphor. Will in its proper sense is called the will of good pleasure; and will metaphorically taken is the will of expression, inasmuch as the sign itself of will is called will.
Reply to Objection 1. Knowledge is not the cause of a thing being done, unless through the will. For we do not put into act what we know, unless we will to do so. Accordingly expression is not attributed to knowledge, but to will.
Reply to Objection 2. Expressions of will are called divine wills, not as being signs that God wills anything; but because what in us is the usual expression of our will, is called the divine will in God. Thus punishment is not a sign that there is anger in God; but it is called anger in Him, from the fact that it is an expression of anger in ourselves.
Source: Summa Theologica (New Advent)