74 So the nature of Christ needed no change, or question, or answer, that it should advance from ignorance to knowledge, or ask of One Who had continued in silence, and wait to receive His answer: but, abiding perfectly in mysterious unity with Him, it received of God its whole being as it derived from Him its origin. And, further, it received all that belonged to the whole being of God, namely, His knowledge and His will. What the Father knows, the Son does not learn by question and answer; what the Father wills, the Son does not will by command.
Since all that the Father has, is His, it is the property of His nature to will and know, exactly as the Father wills and knows. But to prove His birth He often expounds the doctrine of His Person, as when He says, I came not to do My own will, but the will of Him that sent Me. He does the Father's will, not His own, and by the will of Him that sent Me, He means His Father. But that He Himself wills the same, is unmistakeably declared in the words, Father, those whom You have given Me, I will, that, where I am, they also may be with Me. The Father wills that we should be with Christ, in Whom, according to the Apostle, He chose us before the foundation of the world, and the Son wills the same, namely that we should be with Him.
His will is, therefore, the same in nature as the Father's will, though to make plain the fact of the birth it is distinguished from the Father's.
Source: On the Trinity (New Advent)