Objection 1. It would seem that the goodness of the human will does not depend on its conformity to the Divine will. Because it is impossible for man's will to be conformed to the Divine will; as appears from the word of Isaiah 55:9: "As the heavens are exalted above the earth, so are My ways exalted above your ways, and My thoughts above your thoughts." If therefore goodness of the will depended on its conformity to the Divine will, it would follow that it is impossible for man's will to be good. Which is inadmissible.
Objection 2. Further, just as our wills arise from the Divine will, so does our knowledge flow from the Divine knowledge. But our knowledge does not require to be conformed to God's knowledge; since God knows many things that we know not. Therefore there is no need for our will to be conformed to the Divine will.
Objection 3. Further, the will is a principle of action. But our action cannot be conformed to God's. Therefore neither can our will be conformed to His.
On the contrary, It is written (Matthew 26:39): "Not as I will, but as Thou wilt": which words He said, because "He wishes man to be upright and to tend to God," as Augustine expounds in the Enchiridion [Enarr. in Ps. 32, serm. i.]. But the rectitude of the will is its goodness. Therefore the goodness of the will depends on its conformity to the Divine will.
I answer that, As stated above (Article 7), the goodness of the will depends on the intention of the end. Now the last end of the human will is the Sovereign Good, namely, God, as stated above (1, 8; 3, 1). Therefore the goodness of the human will requires it to be ordained to the Sovereign Good, that is, to God.
Now this Good is primarily and essentially compared to the Divine will, as its proper object. Again, that which is first in any genus is the measure and rule of all that belongs to that genus. Moreover, everything attains to rectitude and goodness, in so far as it is in accord with its proper measure. Therefore, in order that man's will be good it needs to be conformed to the Divine will.
Reply to Objection 1. The human will cannot be conformed to the will of God so as to equal it, but only so as to imitate it. In like manner human knowledge is conformed to the Divine knowledge, in so far as it knows truth: and human action is conformed to the Divine, in so far as it is becoming to the agent: and this by way of imitation, not by way of equality.
From the above may be gathered the replies to the Second and Third Objections.
Source: Summa Theologica (New Advent)