The second son stayed at home but is also “outside of the house,” which is Jesus’ way of showing that he also was not in fellowship with his father, even though he was close to the house.
The second son was furious at the grace the father showed to the returning prodigal, which Jesus says means that he doesn’t get that he also is a recipient of grace. He thinks he deserves the Father’s possessions, and he never deals with the core issues of his heart—pride and his need for grace—and these things keep him separated from the father, just like the prodigal.
Jesus was saying that there are two ways to be separated from God.One is by defying the laws of God (like the prodigal son). The second is in thinking you are good enough to earn the Father’s approval, never dealing with the core of your heart, and failing to recognize the grace that you desperately need (like the second son).
It’s kind of like a married guy who keeps a mistress. He’s with his family most days, but Friday and Saturday mornings he spends with his mistress. When his wife gets upset with him, he really tries to focus on being a great husband Sunday through Thursday. But he doesn’t stop seeing the mistress on Friday and Saturday.
The problem is not that he’s not trying hard enough on the days he is with his wife. The problem is that he has a divided heart that he’s given to another lover.
That’s what religion does. It keeps a lot of people from ever dealing with the root problem in their heart: the love of self, money and idols rather than love of God.
Even worse, religion keeps you from throwing yourself on God’s grace, the only hope you have of heaven.
This is why John is so harsh with the Pharisees and Sadducees. He wants to wake them up. His message for religious people in Jesus’ day is the same for people today who spend all their time trying to figure out what other religious thing they need to do to be a good Christian:
Stop doing your damnable good works, and just repent. Surrender yourself fully to God, and throw yourself on his grace.
It is our only hope.