What is God's glory?
Wow. That's a good question, because we talk about it endlessly, don't we? And we should know what we're talking about. And yet it is very difficult to define. I'll make a stab at it.
The reason it is so important is because in the Bible I don't know of any truth that is more fundamentally pervasive than God's zeal to be glorified, which means his zeal for us so to think, so to feel, and so to act as to make him look as glorious as he is. We don't add to his glory.
So we want to make God's glory shine. We want to make it visible. "Let your light so shine before men that they may see your good deeds and glorify your Father who is in heaven" (Matthew 5:16). So the goal of my life should be to so live that when people know me well enough, they would say, "God is glorious!" Not "John is glorious," but "God is glorious!" (Which is probably why God lets us sin as much as he does. But that's another question.)
What is it? I believe the glory of God is the going public of his infinite worth. I define the holiness of God as the infinite value of God, the infinite intrinsic worth of God. And when that goes public in creation, the heavens are telling the glory of God, and human beings are manifesting his glory, because we're created in his image, and we're trusting his promises so that we make him look gloriously trustworthy.
The public display of the infinite beauty and worth of God is what I mean by "glory," and I base that partly on Isaiah 6, where the seraphim say, "Holy, holy, holy is the Lord God Almighty. The whole earth is full of his—" and you would expect them to say "holiness" and they say "glory." They're ascribing "Holy, holy, holy is the Lord God Almighty. The whole earth is full of his—" and when that goes public in the earth and fills it, you call it "glory."
So God's glory is the radiance of his holiness, the radiance of his manifold, infinitely worthy and valuable perfections.