Why must God reach the fullness of Glory (reformed theology)?

All reformed theology adheres to is that the purpose for anything created is to 'glorify God' but that is obviously true under any theology that uses reason within its framework. God must have a reason for making something (beyond his single own existence) and the reason must be within himself (according to his own independent existence) before anything was created because there was nothing else to base a reason on. Therefore the question is what reason did God have within himself whereby he decided to create something. The bible gives the answer his 'glory' and the general goodness of his will according to that infinite glory.

So let's rephrase it. Why would God emanate or multiply his own infinite glory into an act of creation, whereby the outcome would result in God with his infinite glory and a creature that reflects that glory? In mathematics it's like asking why would infinity create the finite? It's an infinity plus one scenario. God saw fit and took joy in the idea of creating a small bit of his glory reflected in a creation because he saw infinity plus one as more than infinity. In one sense Gods glory can't increase but in another sense his overall glory in the entire existence of all creatures does increase after creation because in some sense infinity plus one is greater than infinity, although in another sense it never can be (infinity plus one = infinity).

It's true everyone refers to Jonathan Edwards as he expresses the ideas so clearly without adding foolish ideas that take away from the infinite glory of God.

4. As there is an infinite fulness of all possible good in God—a fulness of every perfection, of all excellency and beauty, and of infinite happiness—and as this fulness is capable of communication, or emanation ad extra; so it seems a thing amiable and valuable in itself that this infinite fountain of good should send forth abundant streams. And as this is in itself excellent, so a disposition to this in the Divine Being, must be looked upon as an excellent disposition. Such an emanation of good is, in some sense, a multiplication of it. So far as the stream may be looked upon as any thing besides the fountain, so far it may be looked on as an increase of good. And if the fulness of good that is in the fountain, is in itself excellent, then the emanation, which is as it were an increase, repetition, or multiplication of it, is excellent. Thus it is fit, since there is an infinite fountain of light and knowledge, that this light should shine forth in beams of communicated knowledge and understanding; and, as there is an infinite fountain of holiness, moral excellence, and beauty, that so it should flow out in communicated holiness. And that, as there is an infinite fulness of joy and happiness, so these should have an emanation, and become a fountain flowing out in abundant streams, as beams from the sun. Thus it appears reasonable to suppose, that it was God’s last end, that there might be a glorious and abundant emanation of his infinite fulness of good ad extra, or without himself; and that the disposition to communicate himself, or diffuse his own fulness,* was what moved him to create the world. (The Works of Jonathan Edwards with a memoir by SERENO E. DWIGHT, P100)

Do you see the infinity plus one concept? So if creation is 1, our purpose is to be in uniformity to God's nature and excellence so that God's overall glory of infinity plus one is obtained in the combination of God and the created. This creation was in order to multiply God's original perfection and excellence in a sense more than previously existed (infinity plus one), without concluding anything lacking originally in God (infinity).

Note: A creatures perfect joy and happiness is necessarily included in this conformity of glory , that is why everyone is rejoicing in heaven because they are indeed reflecting his excellent wonder.

Let me answer a few of your side-concerns before I get to your main question.

1. Reformed theology stresses that God created the universe for one purpose: to glorify him fully

   I'm not certain how you're using the phrase "glorify him fully", but it could be interpreted to mean that God felt that he wasn't being glorified enough prior to creation, and therefore created the universe and ordained all events so that in the end he could finally get the glory he had been missing.

   That may not be what you're saying at all, but let me just clear it up in case you're wondering about that. Here's John Piper:

   This is why God created the world — “that he may be glorified.” Which does not mean: “that he may be made glorious.” Don’t take the word “glorify” and treat it like the word “beautify.” To beautify means to take a plain room and make it beautiful. We don’t take a plain God and make him beautiful. That is not what glorifying God means.

   When God created the world he did not create out of any need or any weakness or any deficiency. He created out of fullness and strength and complete sufficiency. As Jonathan Edwards said, “Tis no argument of the emptiness or deficiency of a fountain that it is inclined to overflow” (Yale: Works, Vol. 8, 448). So we don’t glorify God by improving his glory, but by seeing and savoring and showing his glory (which is the same as knowing, loving, showing).

   and Jonathan Edwards:

   [Nothing would] imply any indigence, insufficiency, and mutability in God; or any dependence of the Creator on the creature, for any part of his perfection or happiness. Because it is evident, by both Scripture and reason, that God is infinitely, eternally, unchangeably, and independently glorious and happy: that he cannot be profited by, or receive anything from, the creature; or be the subject of any sufferings, or diminution of his glory and felicity, from any other being.

2. The theology makes God out to be self-obsessed.

   Self-love is an ugly thing in sinful creatures. But consider who God is. He's the most perfect, wonderful, pure, holy, sinless, morally good, and valuable being in existence. And for that reason, it is completely appropriate for God to love himself above everything else.

   When you and I act in a humble, self-deprecating manner, we are acting in a way appropriate to our sinful nature. But there is no cause for God to be humble or self-deprecating - in fact, it would be entirely inappropriate for him to act as if there was anything higher, greater or more praiseworthy than himself. Since there is nothing more glorious than God, nothing should receive greater praise than God... including from God himself.

3. Now, to answer your question, it is correct to say that God created the universe for his own glory--that is, to receive recognition, honor, and delight from mankind. From your question it sounds like you already agree that the Bible makes that clear, so I won't focus on proving that. If you'd like to see that in the Bible, try passages like Isaiah 43:6-7, Ephesians 1:5-6, Isaiah 40:4-5, Romans 1:20-21, and Philippians 1:20. I can give you more if that would be helpful.

   But where many people go wrong is in assuming that God acts like a narcissistic dictator, demanding worship from people who are either groveling slaves or simpering sycophants, and smiting them in wrath if they don't. But that's not the right way to think of God's glory at all. Here's Sereno Dwight, writing about Jonathan Edwards:

   From the purest principles of reason, as well as from the fountain of revealed truth, he demonstrates that the chief and ultimate end of the Supreme Being, in the works of creation and providence, was the manifestation of his own glory in the highest happiness of his creatures.

   John Piper, who I've quoted before and is one of the pre-eminant reformed pastors of our day, has centered his life's work on this realization that God's glory has a direct bearing on our happiness. His most famous statement, and the underpining of "Desiring God Ministries" is:

   God is most glorified in us when we are most satisfied in him .

   One last quote on this from Jonathan Edwards:

   God in seeking his glory seeks the good of his creatures, because the emanation of his glory... implies the... happiness of his creatures. And in communicating his fullness for them, he does it for himself, because their good, which he seeks, is so much in union and communion with himself. God is their good. Their excellency and happiness is nothing but the emanation and expression of God’s glory. God, in seeking their glory and happiness, seeks himself, and in seeking himself, i.e. himself diffused... he seeks their glory and happiness.

   Thus it is easy to conceive how God should seek the good of the creature... even his happiness, from a supreme regard to himself; as his happiness arises from... the creature’s exercising a supreme regard to God... in beholding God’s glory, in esteeming and loving it, and rejoicing in it.

   One of the implications of this is that refusing to seek the glory of God is not only sinful in the light of who God is - it is self-destructive of a person's own happiness.

   Be appalled, O heavens, at this; be shocked, be utterly desolate, declares the LORD, for my people have committed two evils: they have forsaken me, the fountain of living waters, and hewed out cisterns for themselves, broken cisterns that can hold no water. (Jeremiah 2:12-13)

So to conclude, the Bible teaches that God's glory -- the praising and honoring of the greatest being imaginable -- is the very basis for human happiness. Therefore in creating the universe for his own glory, God gives the greatest gift to those who glorify him - the gift of himself. God's didn't create the world because he needed our praise. Rather, he is completely satisfied and delighted in himself, and in the overflow of his delight he created us so that we too could glorify him, be delighted in him, and find in him our greatest happiness and satisfaction. The most loving thing God can ever do for a person is to give that person a sense of God's own infinite worth.

Here are some references. I highly commend the first one two you. If you prefer to jump straight to some of the objections that could be raised against God seeking to glorify himself, you might want to go right to page 162.

God's Passion For His Glory (the free PDF link is under the book cover)

Why Did God Create the World?

How Does It Glorify God to Predestine People to Hell?

Is God a Monster?

God Wants to Show His Wrath (Romans 9:22–23, Part 1)

The Ultimate Purpose of the Universe (Romans 9:22-23, Part 2)