Concerning the energies in our Lord Jesus Christ
But if any one should say, “We do not say that Christ has but one nature, in order to do away with His human energy, but we do so because human energy, in opposition to divine energy, is called passion (πάτθος),” we shall answer that, according to this reasoning, those also who hold that He has but one nature do not maintain this with a view to doing away with His human nature, but because human nature in opposition to divine nature is spoken of as passible (παθητική). But God forbid that we should call the human activity passion, when we are distinguishing it from divine energy. For, to speak generally, of nothing is the existence recognised or defined by comparison or collation. If it were so, indeed, existing things would turn out to be mutually the one the cause of the other. For if the human activity is passion because the divine activity is energy, assuredly also the human nature must be wicked because the divine nature is good, and, by conversion and opposition, if the divine activity is called energy because the human activity is called passion, then also the divine nature must be good because the human nature is bad. And so all created things must be bad, and he must have spoken falsely who said, And God saw every thing that He had made, and, behold, it was very good.
We, therefore, maintain that the holy Fathers gave various names to the human activity according to the underlying notion. For they called it power, and energy, and difference, and activity, and property, and quality, and passion, not in distinction from the divine activity, but power, because it is a conservative and invariable force; and energy, because it is a distinguishing mark, and reveals the absolute similarity between all things of the same class; and difference, because it distinguishes; and activity, because it makes manifest; and property, because it is constituent and belongs to that alone, and not to any other; and quality, because it gives form; and passion, because it is moved. For all things that are of God and after God suffer in respect of being moved, forasmuch as they have not in themselves motion or power. Therefore, as has been said, it is not in order to distinguish the one from the other that it has been named, but it is in accordance with the plan implanted in it in a creative manner by the Cause that framed the universe. Wherefore, also, when they spoke of it along with the divine nature they called it energy. For he who said, “For either form energises close communion with the other,” did something quite different from him who said, And when He had fasted forty days, He was afterwards an hungered: (for He allowed His nature to energise when it so willed, in the way proper to itself,) or from those who hold there is a different energy in Him or that He has a twofold energy, or now one energy and now another. For these statements with the change in terms signify the two energies. Indeed, often the number is indicated both by change of terms and by speaking of them as divine and human. For the difference is difference in differing things, but how do things that do not exist differ?
Source: Exposition of the Faith (New Advent)