Keller says that churches are typically filled with older brother types, who have always been responsible and hard-working. Going to church seems like the right thing to do to them, so they fill the pews. But the younger brothers (the rebellious, counter-culture types) often feel like they don't belong in church, partly because the older brothers don't make them feel welcome.
The Prodigal God is about God's love for both older and younger brothers and sisters. In fact, Keller points out that the word "prodigal" means "extravagant"--hence the title. God is indeed extravagant in his love for all of us.
I want to focus on the father's words to his angry oldest son, who complained that he had served his father his entire life, and had never been able to celebrate with his friends. The father replied, "Son, you have always been with me, and all that is mine is yours." (Luke 15:31) Everything the father had belonged to him, and yet he lived as a slave? He'd never had a party? Why?
This man didn't really understand the meaning of grace. The rebellious, extravagant, hard-living younger brother was a better Christian than he was for two reasons: First, when he came back to the father, he admitted his unworthiness and left his sins behind. Galatians 5:20-21 says that you can't be a real Christians and keep abusing drugs, engaging in promiscuous sex, etc., and I think we all agree on a visceral level. Maybe the younger brother did those things, but he left that lifestyle behind and came back to his father in humility.
But can you be a real Christian and never give up your jealousy, divisive attitude, and outbursts of anger? Oh, sure! Those are understandable sins, and we all struggle with them, right? Well, maybe we do, and that's why they are so insidious. But to God, they are just as serious. In fact, those sins are mentioned along with the younger brother's sins in Galations 5:20-21. But unlike the younger brother, the older brother was not willing to let them go. He didn't understand that grace means surrendering our sins. It doesn't mean overcoming them in our own strength--that's impossible--but letting God deal with them.
The second reason why the older brother didn't understand grace is because he thought it was all about outward service. He figured he deserved credit for his "responsible," "good" behavior. But he really wasn't so good, was he? His heart was corrupt, and as a result his life was joyless. Grace is about receiving what our extravagant Lord wants to give--love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self-control. (Galations 5:22) He wants to forgive us, heal us, provide for us, answer our prayers, and allow us to reign eternally with him. In short, he wants to give us everything that is his. The younger brother was willing to humbly receive, even though he knew that he deserved nothing.
We so often hold on to our filthy rags--our own self-righteousness--that we effectively shut God out. We can never impress him with our own goodness. (Luke 18:9-14) Like the younger brother, we have to come to him as beggars, or not at all. And when we do, God will receive us with a celebration, like the father did in the parable.