Luke 18 and the tireless widow
In the story the widow's future is in the hands of the wicked judge. Her hunger for justice (vindication) against her enemies finally gains for her what she hungers for. The point we need to stress is her refusal to settle for less and so her constant coming because we're told that that's why the story was told, "that they should always pray and not give up." We're shown a woman who wouldn't give up! She wouldn't give up even though she had to deal with a judge that cared nothing for justice. It's because a host of people believe they can't get justice in the courts that they don't report crimes or they sink into despair or, in some cases, become vigilantes.
God's elect (Luke 18:7) are to pay attention to the woman who wouldn't give up even in the face of circumstances that would make it look like there wasn't any point in going on.
Unlike the judge who has no commitment to justice God is wholly committed to it. The slow appearance of justice for the elect is not due to anything in God that is like the judge. [He won't give the elect what they ask just so he can have peace and quiet.]
OT textual background to the parable
The background to the story and the situation that led to the story, without doubt, would be passages like Malachi 2:17 and 3:1-6. The implication in Malachi 2:17 is that God is like the unjust judge of the Luke 18 parable—he shows no obvious interest in justice, which is why we have the sneering question by the ungodly in the Malachi text—the words that God said wearied him, "Where is the God of justice?". If you put the question in the mouths of the wicked (as we clearly should do) it would mean they weren't afraid of the God of justice. If you put the question in the mouths of the Jewish oppressed—a question such people ask in other texts—it would mean that they were near despair.
Malachi 3 insists that God will indeed come in keeping with his commitment to righteousness. He would come in and as the "messenger of the covenant" for whom Israel looked (3:1-3) but when he comes it won't be all joy because those he'll meet will generally be faithless people. Here's what Malachi 3:5 says: "So I will come near to you for judgement. I will be quick to testify against the sorcerers, adulterers and perjurers, against those who defraud laborers of their wages, who oppress the widows and the fatherless, and deprive aliens of justice, but do not fear me, says the Lord Almighty."
Notice the things Malachi has in common with the Luke 18 text. There's the similarity in terms and setting such as, "perjury," oppression, justice withheld, widows, judges (implied) who respect neither God nor aliens, whom they deprive of their due.
In addition to the court terms and setting there's the notion of delay in Malachi 2:17 and throughout the story Jesus tells, and specifically in Luke 18:7. Finally, there's the coming of the Lord in the person of the Messenger of the Covenant and the coming of the Son of Man (Malachi 3:1-3 and Luke 18:8).
Some interpretive pointers from Jesus
Jesus centres the story around justice not yet given and not simply some blessing we'd like to have or obtain.
[I'm thinking of our asking God for healing for ourselves or our loved ones, or a better job or a cure for a hurting marriage. Other texts encourage such requests but this text isn't one of them.]
Jesus clearly implies that the request for justice is one that has been going on for quite some time—it has been delayed (see 18:7).
[Christ's remark about God while it assures the praying people that they will get justice nevertheless indicates that it has been some time in coming. "Will he keep putting them off?" See the comments above on Malachi.]
Jesus parallels this individual in the story to the elect (plural) and the prayers of the elect for justice.
[Certainly the elect is made up of individuals but "the elect" is more than a collection of persons—it is a plural unity, a community, a people. In the application of the story Jesus tells us that he is speaking of "the elect" and the prayers of the elect for justice (in the face of oppressors).]
Jesus insists that despite the delay justice was coming and it was to come soon.
["Will he (God) keep putting them off? I tell you he will see that they get justice, and quickly." Luke 18:7-8; I'm following the NIV and numerous other versions that render similarly. We mustn't dismiss this double assurance. Not only will the elect get it, they will get it "quickly" (the versions agree on that.) Bearing in mind that Jesus said that vindication of the elect would come soon we need to insist on an historical setting.]
Jesus clearly associates the soon coming vindication of the elect with the coming of the Son of Man.
[There are several possible and reasonable understandings of the "coming" of the Son of Man. His yet-future and final coming is hardly one of them since Jesus spoke those words 2,000 years ago. There is the coming of the Son of Man in judgement on the Jewish nation, culminating in 70AD (see Matthew 24:26, 30, 34, Luke 17:22-37). There is the coming of the Lord in the person of the Holy Spirit at his exaltation, when he was made Lord and Judge of all (see John 14:16, 18, 23, 28-29, Acts 2:33-36). All these comings are distinct in reference and should be held as distinct but they're interrelated and are manifestations of the one King of Kings and his authority. You might wish to read this.]
What I'm sure we're not to do with the section is to personalise it and make it a promise that God will give us anything we ask if we just keep asking for it. There is a copper-bottomed assurance in this text that God would grant—and soon—the vindication his elect sought for but we're not to ignore the very words of the text, the section's historical setting and the vast issue in Jesus' mind.
We're not to reduce this divine promise to "vindicate the elect" to God's promise to give us (even good) things that we judge important and for which we hunger. Let me repeat, if I ask God for the recovery of a loved one from debilitating disease (maybe rheumatoid arthritis) there is no guarantee that the loved one will recover and there's certainly no assurance that they will recover soon. This is not what the section is about.
Teachers only injure people when they make promises in God's name that God did not make! This is especially true when what the person earnestly and patiently seeks doesn't come about. In the case above, the agonised arthritic sufferer gets worse and finally dies—what then of our use of this section?
It's our tendency to take all passages on prayer as promises to each and every individual and concerning all of our personal needs and wants. It's hardly surprising then that in so much of the evangelical world prayer becomes self-centred; we encourage it. It also becomes a source of resentment toward God when it appears that God isn't living up to his promises. Sometimes it generates great feelings of guilt for desperate people begin to think that God isn't answering their prayers because they are unworthy. This only adds a great burden to heavy burdens already existing.
This isn't a fair treatment of the text nor is it helpful to a sometimes desperate believer who feels he/she has come to the end of their tether only to find God doesn't keep his promises (the promises we say he made).