What is God's Purpose?

"...when we do ask about God's design, we are too prone to describe it with ourselves at the center of God's affections. We may say, for example, that His design is to redeem the world. Or to save sinners. Or to restore creation. Or the like.
But God's saving designs are penultimate, not ultimate. Redemption, salvation, and restoration are not God's ultimate goal these He preforms for the sake of something greater: namely, the enjoyment He has in glorifying Himself."

Desiring God by John Piper / Multnomah Books / 2003 / pg. 31

Can I Diminish God's Glory?

"A man can no more diminish God's glory by refusing to worship Him than a lunatic can put out the sun by scribbling the word "darkness" on the walls of his cell. But God wills our good, and our good is to love Him (with that responsive love proper to creatures) and to love Him we must know Him: and if we know Him, we shall in fact fall on our faces."

from C. S. Lewis' The Problem of Pain
Chapter 3, "Divine Goodness,"
New York: Macmillan Company, 1948, p. 41.
Harper, SanFrancisco, 2001.