A.A. Hodge finds that the idea of created holiness necessary to a full understanding of the image of God. He lists three “elements” of the image: “(1.) Man was created like God, as to the physical constitution of his nature–a rational, moral, free, personal spirit….(2.) He was created like God as to the perfection and integrity of his nature. This includes (a) knowledge…[which] is restored when the sinner is regenerated in the grace of spiritual illumination [and] (b.) righteousness and true holiness (Eph. iv. 24), the perfect moral condition of the soul, and eminently of the character of the governing affections and will. (3.) In respect to the dignity and authority delegated to him as the head of this department of creation. Gen. 1. 28” (A Commentary on the Confession of Faith, p. 124).
Hodge realizes that there are objections to created holiness. Here is how he (masterfully, it seems to me) handles such objections (I say so while acknowledging that many of the Reformers and Reformed Orthodox held to a version and/or the language of the donum superadditum, a topic perhaps for another time):
Pelagians have held that a created holiness is an absurdity; that, in order that a permanent disposition or habit of the soul should have amoral character, it must be self-decided–i.e., formed by a previous unbiassed choice of the will itself. They therefore hold that God created Adam simply a moral agent, with all the constitutional facilities prerequisite for moral action, and perfectly unbiassed by any tendency of his nature either to good or evil, and left him to form his own moral character–to determine his own tendencies by his own volition. But this view is not true, because–(1.) It is absurd. A state of moral indifference in an intelligent adult moral agent is an impossibility. Such indifference is itself sin. It is of the essence of moral good that it brings the will and all the affections of the soul under obligation.
(2.) If God did not endow man with a positive moral character, he could never have acquired a good one. The goodness of a volition arises wholly from the positive goodness of the disposition or motive which prompts it. But if Adam was created without a positive holy disposition of soul, his first volition must have either been sinful from defect of inherent goodness, or at best indifferent. But it is evident that neither a sinful nor an indifferent volition can give a holy moral character to whatever dispositions or habits may be consequent upon it.
(3.) The Scriptures teach that Adam was created “in righteousness and true holiness.” (a.) God proclaimed all this works “very good.” But the “goodness” of a moral agent essentially involves a holy character.
(b.) Eccles. vii. 29: “God made man upright, but they have sought out many inventions.”
(c.) In Genesis it is declared that man was created in “the image of God.” In Eph. iv. 24 and Col. iii. 10, men in regeneration are declared to be recreated in “the image of God.” Regeneration is the restoration of human nature to its pristine condition, not a transmutation of that nature into a new form. The likeness to God which was lost by the fall must therefore be the same as that to which we are restored in the new birth. But the latter is said to consist in “knowledge, righteousness and true holiness.”
(4.) Christ is the model man (1 Cor. xv. 46, 47), produced by immediate divine power in the womb of the Virgin, not only without sin, but positively predetermined to holiness. In his mother’s womb he was called “that holy thing.” Luke i. 35.
4th. That God should have furnished Adam with sufficient knowledge for his guidance is necessarily implied in the fact that Adam was a holy moral agent and God a righteous moral governor. Even his corrupt and degenerate descendants are declared to have in the law written upon the heart a light sufficient to leave them “without excuse.” Rom. i. 20; ii. 14, 15. Adam moreover enjoyed special and direct revelation from God, and was particularly directed as to the divine will with respect to his use of the fruit of the tree of knowledge of good and evil….
5th. That Adam, although created holy and capable of obedience, was at the same time capable of falling, is evident from the event. This appears to have been the moral condition in which both angels and men were created. It evidently was never intended to be the permanent condition of any creature. It is one, also, of the special elements of which we can have no knowledge, either from experience or observation. God, angels and saints in glory are free, but with natures certainly and infallibly prompting them to holiness. Devils and fallen men are free, with natures infallibly prompting them to evil. The imperfectly sanctified Christians is the subject of two conflicting inherent tendencies, the law in the members and the law of the Spirit; and his only security is that he is “kept by the power of God through faith unto salvation.” (pp. 124-7)