It is helpful to draw a distinction between three things. (i) “personhood,” referring to those ontological features in virtue of which someone is a someone rather than merely something, e.g. rationality or the capacity for moral agency, (ii) “personal identity,” referring to those features of someone that distinguish him from all other persons as a unique someone in an incommunicable manner, and (iii) “personality,” referring to the full development of the features in both (i) and (ii) into the maturity that we experience now to a degree, but ultimately, is only brought to full perfection in the eschaton.
In the above sense, personhood is common and shared, while personal identity is not. Both Peter and Paul are “individual substances of a rational nature,” both possess free will or the dignity proper to personal moral agents or whatever else we want to say are the distinguishing features of persons as opposed to things. Such an ontological basis sets Peter and Paul apart from trees and rocks, but does not distinguish between Peter and Paul. Such features tell us what we are as persons, but they do not tell us who we are as this person. Granted, a particular instance of a general feature such as rationality is, as an instance, a particular instance metaphysically distinct from other instances. So Peter’s rationality is metaphysically distinct from Paul’s just as Peter’s animality is metaphysically distinct from Paul’s. Nevertheless, when we identify the unique and irreplaceable character of Peter’s personal identity, we are identifying more than the metaphysical uniqueness that a particular pebble has—mysterious and wonderful as the pebble’s particularity may be in its own right.
This distinction between personhood and personal identity may lead our minds toward those features of Peter that make him stand out from others in our experience: his brashness, his all too ready devotion to our Lord, his bearded visage. But this would be a mistake. In the first place, such qualitative features are, in principle, sharable with others. As the scholastics say, they are communicabilis. While Peter’s brashness makes him stand out from the crowd, other persons can be brash too. If the brashness were what made Peter Peter, then the others would be just as much Peter as he. Indeed, if they were more brash they would be more Peter than Peter. We might appeal to the idea that it is not just the brashness alone, but the combination of brashness and beard, and perhaps many other qualitative traits, but such a scheme would turn Peter into nothing more than an aggregate of communicable traits, even if we found some reason to think that the aggregate as a whole would be unique.1
Understanding Peter’s identity in terms of his striking features or idiosyncrasies faces an even graver problem, however: Peter was not always brash, was not always bearded, and we do not know whether he will retain these qualities in the future. From the moment of his conception, Peter was Peter, an irreplaceable person loved by God as this unique beloved. Hence, Peter’s personal identity cannot depend on these acquired features. And while it may depend ontologically on those features we previously identified as essential to his personhood, it cannot be wholly constituted by them. Instead, we are left with a mysterious remainder, a dimension that goes beyond those features shared by Peter and Paul, yet prior to those features that Peter acquires in the course of his life. In the dark of those first hours as an embryo, Peter is already endowed by God with the dignity of personhood, but he is also endowed with the dignity of being Peter, and the exact character of that dignity in those dark hours can be known only to the One who granted it.
But this is not the end of the story. From the first moments of his life, Peter also receives a call. He does not remain an embryo, and his destiny is more than the bare fact of his personhood or even the bare fact of his unique personal identity, wonderful as these may be. Hence, if I may be allowed to put it this way, we may distinguish between his “peterness” as an endowment and his “peterness” as an achievement as he lives up to the unique call placed upon his life. Notice that the former is not something that he could fail to receive or ever lose, but the latter is something that he must live up to through his free moral agency as he cooperates with grace. This latter full development of all that it means to be Peter, is what I call his “personality.”
Now, the concept of Peter becoming Peter in this last sense is not just the same concept as Peter becoming a saint, at least not in an obvious way. But the connection between the one and the other is rich, as CS Lewis famously remarks in Mere Christianity:
It is no good trying to “be myself” without Him. The more I resist Him and try to live on my own, the more I become dominated by my own heredity and upbringing and surroundings and natural desires. In fact what I so proudly call “Myself” becomes merely the meeting place for trains of events which I never started and which I cannot stop. What I call “My wishes” become merely the desires thrown up by my physical organism or pumped into me by other men’s thoughts or even suggested to me by devils… Until you have given up your self to Him you will not have a real self. Sameness is to be found most among the most “natural” men, not among those who surrender to Christ. How monotonously alike all the great tyrants and conquerors have been: how gloriously different are the saints.2
Responding to the holiness of God and thereby growing in holiness ourselves is the surest way to grow into the fullfidness of our personality. By contrast, responding to our impulses, passions, and pride leads to a loss of personality, although our most fierce rebellion can never utterly erase that kernel of personal dignity with which we are endowed by sheer grace from the beginning.