Because, you know, it’s that time of year. But, of course, the topic is never out of season.
There was a dilemma involved in the restoration of man from sin. Only man was obligated to pay what was owed to God, but man was unable to do it. The Son of God was willing to pay the debt, but could not as unincarnate, for God is by nature immortal. So the Son of God assumed human nature to pay the debt, being able “through His divine nature to do what was required.”
In Christ the diversity of natures and the oneness-of-person served the following end: If the human nature was not able to do what was required to be done for restoring men, then the divine nature would do it; and if [what was required] did not at all befit the divine nature, then the human nature would do it. And not two different [individuals] but one and the same [individual], existing perfectly in two natures, would pay through His human nature what this nature ought to pay and would be able through His divine nature to do what was required. (Cur Deus Homo 2.17, tr. Jasper Hopkins and Herbert Richardson)
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