Q: Why did God make you and all things?
A: For His own glory.
This question and answer pair comes from the children's catechism that my church and my family are using. It is a venerable answer, deriving from the first question of the Westminster Shorter Catechism, which reads:
Q: What is the chief end of man?
A: Man's chief end is to glorify God, and to enjoy him forever.
One thing that stands out in the transition from the standard answer to the children's answer, however, is the change in subject. The original answer is (directly) about man, and the children's answer is exclusively about God. This raises a question, I think. It is obvious that putting the glory of God as our highest purpose, from the human vantage point, makes excellent sense. It is less obvious what it means, however, to say that God's motive is His own glory.
When we say that God does something "for His glory," what precisely do we mean? We must admit, from the perspective of classical Christian theology, that this can't mean any change in God's own property of glory. God does not become in Himself more or less glorious under any circumstances. The Lord does not change. The objective character of God's glory is forever constant. Nothing can add to or take away from it.
So for God to act for His glory seems to require that this refer to the relation between His objective glory and something else. One might suggest that this "something else" is God's self-perception, that God acts to better see and enjoy His own glory. Yet this cannot be either, as God's self-perception is itself infinite and immutable, eternally full and complete.
For God to act for His glory, then, must refer to the relation between His glory and something outside of Himself. What could that be? The most obvious option would be human perception. To say God acts for His glory, then, would mean that He acts so that humans will subjectively perceive His objective glory. We could then paraphrase the catechism answer to the question "Why did God make all things?" as, "So that we would be able to see His glory."
But I would like to press further. Why would God wish to create human subjects to perceive His glory? The answer can't be that God needs to so enhance His objective glory with its subjective perception, as that would make God dependent on the world. Nor can we say He needs this to be happy, as though He is not full of life and beatitude and blessedness in Himself, apart from anything He may or may not create. We must in fact avoid attributing any kind of lack to God that would require Him to glorify Himself before men. He is not served by human hands, and He is who He is in perfect Trinitarian fellowship with or without us.
This leads us to a fundamental wall. Whatever God intended in creating all things, it cannot be understood as motivated by anything He wishes to receive. God did not create in order to fill Himself up, for He is the fullness of life in Himself already. If God's will in creation is to be understood at all, we must look in outward terms. The only way to make any sense of this without attributing need or lack of God is to think in terms of overflow and gift. Even here we strain the limits of human understanding, but the path seems clear. God's creative activity seems only conceivable in terms of His will to share and bless, to amplify His own beauty and blessedness by replicating it on smaller scales, for others to see and enjoy as He already does.
How do we fit this with God creating all things for His own glory? We might paraphrase the answer to the question like this, "To reveal His glory to people who will be blessed by it, so that they share in God's own life and beatitude." This is a very different take than what sometimes goes around in Calvinistic circles, where claims are made to the effect that God just does what God wants because He loves being praised so much for His own pleasure. But God needs no such thing. God is full of infinite blessedness on His own already. The only party who stands to gain from the revelation of God's glory is the creaturely one. We come alive in the light of the One in whom we live and move and have our being. God's glory is the food of our eternal happiness, in a small reflection of how this is for God Himself.
So when God acts to create, He does act for His glory. He acts to unveil and reveal His glory, that He might share life and true happiness with creatures who live by this splendorous vision. He has nothing to gain, but we have everything to gain. This is a fundamentally gracious motivation, a free gift. Indeed, the right word is probably "love." The God who is love creates all things in the freedom of this love, as sheer gift, creatio ex gratia.