Though we often tend to be idealists when it comes to the Christian life, Jesus is not. We want to say, "God will take care of you" or "It will all come out right in the end" or "Everything happens for a reason." While those statements may be theologically correct, the life of faith doesn't always feel like the Right Answers.
Jesus is a realist. He understands what life is really like for those who live in a sin-broken world and need to trust daily that God is good and that he is in control of our challenging circumstances, our disappointments, and our unfulfilled longings. That's what this parable is about.
Jesus is not, note, saying that God is actually like this unjust judge. Rather, Jesus is saying that sometimes our experience feels like God is behaving this way. We may feel like we have been storming the gates of heaven in prayer and still we are stuck, frustrated, disappointed, longing. Jesus' answer? Don't quit. Don't give up. Continue to lay your needs before God. Continue to pester God with your needs. Continue to pray, partly at least because the act of praying is an implicit acknowledgement that God is in fact sovereign.
The Bible is full of examples of those who trusted God, often for unbearably long periods of time, to fulfill his promises. How long did Abraham wait for a legitimate heir? How long did David hide in caves? How long did Moses herd sheep in the wilderness?
There are so many great examples of believers who have endured challenging circumstances that tested their faith, but one of the most potent for me is Dietrich Bonhoeffer's collection, Letters and Papers from Prison. As Bonhoeffer was imprisoned by the Gestapo during World War Two, he waited for positive word about the plots he'd helped foment against Hitler. He waited for news about his parents and the rest of his family. He waited to have any contact with his beloved, to whom he had very recently become engaged. Reading his reflections is an amazing pendulum swing between hope and despair, faith and frustration.
If you are in a place of waiting, a place of frustration, a place of longing, ponder Jesus' words here. He's saying even if you are suffering, don't give up. Trusting in God can take the shape of persistent prayer. Allow yourself to believe in the face of the immediate evidence that God is good, that he is bringing his promises to fruition for your good and his glory.