At the end of the sermon, Keller sets up a beautiful picture of Christ being our true elder brother.
"The true elder brother would have seen the agony of the Father and said "Father, I'm going to go out and look for my brother. And if he has ruined himself and squandered his inheritance, I'll bring him home at my own expense.' That would have been a true elder brother. Poor kid, he doesn't have a true elder brother...but we do. Jesus Christ gives us a bad elder brother so we'll long for the right one. We don't just need an elder brother to go out in to the next town to find us, we need someone to come from heaven to earth. We don't need an elder brother who brings us in to God's family just at the cost of wallet, but at the cost of his life. Because on the cross, Jesus Christ was stripped naked so that we could be clothed in the robe of honor that we don't deserve. On the cross, Jesus called 'My God, My God', the only time he never called Him Father, because at that moment He was not being treated as a Son so you and I could be. There He paid the debt that deep down we all know we owe. And because everything He had--He had everything the Father had, but He shares it with us and brings us home at enormous expense to Himself. And when you see that, to the degree you see that, it will change the absolute motivation, your whole approach towards God. And you won't be into self-discovery or moral conformity, you'll be a Christian."Go here to check out more about Tim Keller and get some free sermons. The "Prodigal Sons" sermon is about half-way down the page.