The problem for both sons is their heart toward God. In their mind, God is there for him. They are not there for him. What is needed is a change of heart. Keller writes, "How can the inner dynamic of the heart be changed from one of fear and anger to one of joy, love, and gratitude? Here is how. You need to be moved by the sight of what it cost to bring you home. (P. 87). "...The first thing we need is God's initiating love."
Keller says the older brother is in an analogous position as Jesus. What he should have done is what Jesus did - he should have chased after the brother and sought to rescue him - even at great cost to himself - for it is the older son's inheritance that would be spent on the rescue. Jesus paid the cost for our rescue. He chased us all the way to the depths of sin for the purposes of our rescue. The only way out of being either the older brother or younger brother is to recognize the love of God in pursuing us all the way to the cross. Recognition of his love changes our hearts and transforms our minds. Keller: "When we see the beauty of what he has done for us, it attracts our hearts to him" (p.88). Keller quotes William Cowper:
"To see the law of Christ fulfilled
And hear his pardoning voice,
Changes a slave into a child
And duty into choice" (p. 89)
"We will never stop being younger brothers or elder brothers a until we acknowledge our need, rest by faith, and gaze in wonder at the work of our true elder brother, Jesus Christ." (P. 89)
Our charge as witnesses ourselves is to proclaim what we have seen and heard of our Father's love for us through the Son - to lift us Jesus so people can see and have their hearts transformed. It's good news for all who simply look.