"Sin is what you do when your heart is not satisfied with God. No one sins out of duty. We sin because it holds out some promise of happiness. That promise enslaves us until we believe that God is more to be desired than life itself (Psalm 63:3). Which means that the power of sin's promise is broken by the power of God's" (Future Grace, pg. 10-11).
So...Mr. Piper's thesis is essentially that we sin because we do not believe God's promises will deliver more happiness than the "happiness" that sin promises us. Well, in that case, I suppose there is much unbelief in my life because I sin many times each day. I am definitely looking forward to participating in this study with the ladies in my church. It is always such a joy to seek to apply God's word to our lives and grow together! I'm sure I'll be sharing more little nuggets from this book in the future.