It is often quipped: People change.

This is invariably spoken either by someone who, in the moment, finds it expedient to dismiss irredeemable or unforgivable past behavior, or by someone who has an unexceptionally shallow understanding of the man-creature. Recall Cioran:

"Man is free, save for his depths. On the surface, he does as he likes; down there, will is a meaningless syllable."

In a facile sense, yes, obviously people do change—it is interesting to note that "facile" and "face" share the same etymology—but this pertains only to actions or, in many cases, the actions to which we are privy (to which we are made privy).

The person, in his inmost—regardless of which terminology you find least objectionable; soul, spirit, limbic system and/or midbrain, etc.—is incapable of change. Whether this is due to the cluster or souls from which the person's inmost descended, or the unique combination of genetics ("nature"), and trauma and early-life care ("nurture")—again, pick your particular prejudice—is immaterial. People do not change. All that changes is the rind.

I suspect that it is something uniquely Christian or, more generally, "baptismal" in a broader sense—which I would use to refer to any of the religious and speculative denominations and cults which were wont to dunk adherents in the Eastern Mediterranean c. 300BCE-500CE—that gives the impression that one's inmost might actually, essentially, change. Ostensibly for the better (or, perhaps, closer to theosis). It is vanishingly unlikely that the literal, physical, "general baptismal rite," does not predate all of the above, as Jews (previously Israelites) and likely their contemporaries have been using the act to mark a certain change in a person for many millennia. In the case of Jewish immersion (to avoid the explicit, contemporary, Christian terminology—and to betray my own prejudice), however, it is to mark the formal and legal entrance of the individual into the Jewish nation. It did not until somewhat more recently appear to have any sort of explicitly metaphysical implications for the inmost of the nascent Jew.

Who you are and what you are are radically different from how you act and appear. The former are wholly immutable. The latter are absolutely plastic, so long as you have your wits about you. All that can be reliably controlled are your actions ("deed"); what you believe, deep in your self and your bones ("creed"), is not up to you. Genuine belief is not something one can simply adopt. Of course, there are sudden, nonlinear religious experiences, which often lead to conversion, but these are uncommon events. They are, incidentally, often affected by the aforementioned expedient-types; it is even more often "experienced" by those who have had initiation into nothing above (or below) the immediate—the sensory, the rind.

This is the genuine expression of the reality of free will. Those with a dogmatic, even fanatical dedication to a deterministic, mechanistic cosmos, will point to the brain, invoke a few rather outdated physical principles, and declare, "Behold! The human is an automaton! He is a slave to matter: all that he is and all that he does (read: can do) is determined prior to and beyond—though, there are no beyonds!—even his first inklings of consciousness!" 

This is silly, but so is the man-creature.

There is also the opposite type—which, in truth, is indistinguishable from the above, despite his automatic flailing protestations—who also claims that all is physical (surely! undoubtedly!) but that this physical is utterly nondeterministic, as it is “chaotic.” Though, if you probe his understanding of the term, you will almost certainly be met with a confused morass of mutually-incompatible ideas, but again: the man-creature is silly.

I would be remiss were I not to mention that other obnoxious, queer, and necessarily modern notion that, somehow, nothing is determined and that all is free. This is merely the secularized (read: hollowed-out and unhallowed) incarnation of the above misconception concerning the mutability of the human's inmost. That is to say: instead of an external, metaphysical-theological, "baptismal" event, there is an internal, physical-personal, decision, which is a decision expressing and representing the absolute and unfettered free will of the individual.

But, as in all things, the reality is neither and both—that is, dialetheic, for the philosophically-minded of you; fuzzy and quantum, for the mathematically or physically-minded of you—all of you, needing a metaphors, you sad, sad, man-creatures, you. 

The externalities of man are absolutely free: his actions are under his control (barring severe physiological pathologies which are exceptions proving the rule), while that in him which is deep—which, being a part of the Above, has infinite depth—is unchanging, just as the Deep is unchanging. ("As below, so within," one might be liable to instinctively mutter; I would advise both for and against this.)

The rind is free, ever-shifting, deceptive, social. The inner, glowing fruit—shielded from the raging storm, the great and terrible cloud, and the never-ceasing flame—is fixed, perfect (despite, or perhaps due to, its being shattered, though it refuses to disintegrate into precisely the sum of its parts), and absolutely personal. 

That one might ever change is inconceivable to him who can conceive at all. The term "man-creature" is chosen deliberately. Man may mutate, but the creature within—yet inextricably intertwined with this man-rind—is, as the etymology suggests, a created being. That which, buried within the human, is impossible to see directly (for the annihilating Light is too bright, too terrible) is a creation: despite being a part of the Great Soul above, which is absolutely simple and so without parts or distinctions, is a created entity: a creature. 

Man may contort his face, buy and sell and trade masks, even undertake many painful surgeries of the flesh. But: there is neither contortion nor exchange of souls, and no surgery of the spirit—sincere or affected—will ever make even the minutest of changes to that which lies forever within.