Even though parts of this post may seem tediously pedantic, I am going to try to make it as painfully clear as possible. Still, you may want to read it slowly, and make sure you digest one sentence before gradueating to the next.
Hartshorne mentions the same passage from Kierkegaard that I independently discovered back when, to the effect that "Everything that comes into being proves precisely by coming into being that it is not necessary."
In this context, remember what was said a few posts back about the implicit relationship between necessity and eternity -- that the "unconditionally necessary" and the "always" are essentially synonymous.
This is because to say that something must be (i.e., is necessary) is the same as saying that it cannot possibly not-be (i.e., always is).
Therefore, "a thing is eternal if its 'being' is necessary; and if it is eternal its 'being' is necessary." A corollary is that "no eternal thing exists potentially," again, since an eternal thing must be unconditionally necessary, whereas potentiality means that something may or may not come into existence.
Thus, becoming is never necessary; or, to be perfectly accurate, it is necessary that becoming exist, but its specific contours are left open. In other words, to say that things can only become in one particular way is to be a determinist, and to therefore reduce becoming to necessity.
Now, "Coming into existence is the change of actuality brought about by freedom." Freedom -- if it is really free -- must be indeterminate, right? If it is determined, then we are once again back in the world of necessity and therefore eternity.
This is one reason why predestination must be "vigorously opposed," because it obviously robs man of his freedom and dignity, but more subtly, renders him eternal because necessary.
Frankly, it shouldn't even be necessary to say this, since the whole doctrine of determinism is absurd on its face. Nevertheless, there are people who believe it, both religious and secular (e.g., neurologists who claim free will to be an illusion).
Let's simply follow this line of reasoning where it leads. Everyone has their favorite Bible passages. Bearing in mind that the devil and even Cousin Dupree can quote scripture, one of mine is 2 Corinthians 3:17, "Now the Lord is the Spirit: and where the Spirit of the Lord is, there is liberty."
Indeed, for a Jew, it seems to me that the dominant theme of the Hebrew Bible is exodus and freedom. The same themes obviously infuse Christianity, only presented in a novel and surprising way that no one could have guessed -- and indeed, even had difficulty comprehending retrospectively. In other words, a man nailed to a cross doesn't look like anyone's idea of freedom -- nor escape, for that martyr.
Both God and human beings are persons, which is to say, subjects. God, being the very principle of freedom, must be "wholly free," whereas man, a reflection of the principle, is only "partly so."
In other words, we are a mixture of chance and necessity, of contingency and determinacy. In fact, I prefer to say that we are the adventure of freedom and constraint. The tension between these two sort of creates the drama of life, does it not?
For Hartshorne, "where necessity does not hold, freedom decides." This is consistent with Polanyi's emphasis on how the boundary conditions of a lower level allow for the emergence of the higher (e.g., how the stability of an alphabet allows for the novel emergence of words, words for sentences, sentences for paragraphs, etc., all the way up to Ultimate Meaning).
Here is where things get a little ticklish, but one must not be afraid of the guffah-HA! experience. To be free is to make decisions. This is axiomatic, since freedom shows us an array of potential paths, and we must choose one.
Hartshorne notes that "the merely necessary does not decide, it simply is." Therefore, to the extent that God "decides," then this implies noneternal qualities in God. In other words, he can choose this, or he can choose that. Conversely, if he has no choice, than he simply "is," and is not free.
Hartshorne references Karl Barth, who, according to wikipedia, is considered by many to be "the greatest Protestant theologian of the twentieth century." I can't say I've read much of him, and I don't mention this as an appeal to authority, only to show that there is mainstream precedent for the views we are exploring. And for Barth, "There is a kind of holy change [as well as holy constancy] in God."
This is not paradox, but necessity. We have to invert the traditional understanding, and affirm that to deny God the capacity for "holy change" is to limit him in an all-too-human way.