This parable is a twin of the parable of the neighbor in need (11:5-8). Both are used to illustrate the importance of persistence in prayer. Both present a person in need persistently pressing a request, and both parables call for reasoning from the lesser to the greater: If a neighbor or an unjust judge will respond to the urgent need and repeated request, then will not God also respond? It is an argument from lesser to the greater by which Jesus affirms the faithfulness of God – He will assuredly act on behalf of the righteous.
The widow’s actions are a model of perseverance in the midst of wrong. The literal translation of v.8 is not “faith” as a general category, but is “the faith” – that is the manner of faith demonstrated by the widow. She is certain of God’s justice and thus acts in resolute faithfulness in anticipation of that certainty. The parable is a metaphor for Jesus’ followers who also will encounter hostility, look for the deliverance that accompanies the coming of the reign of God – and not finding it in their lifetime, may become disheartened. Jesus insists that adversity is integral to the process by which God brings salvation (cf. 17:25, 32-34) – and assures his disciples that, despite delay, they are always to be rooted in hope (18:1-18).
This same idea is captured in the Apostolic writing. At the beginning of his missions, Paul fervently prepared the Thessalonians for the imminent arrival of the Second Coming (parousia). There are aspects of his second letter to the Thessalonians by which it seems Paul began to understand that the parousia would be delayed. It did not diminish his fervor for spreading the Good News, but he ever more begins to preach perseverance as a model of faithfulness. Consider Paul’s letter to the community in Rome written near the end of his life:
19 For creation awaits with eager expectation the revelation of the children of God; 20 for creation was made subject to futility, not of its own accord but because of the one who subjected it, in hope 21 that creation itself would be set free from slavery to corruption and share in the glorious freedom of the children of God. 22 We know that all creation is groaning in labor pains even until now; 23 and not only that, but we ourselves, who have the firstfruits of the Spirit, we also groan within ourselves as we wait for adoption, the redemption of our bodies. 24 For in hope we were saved. Now hope that sees for itself is not hope. For who hopes for what one sees? 25 But if we hope for what we do not see, we wait with endurance. (Romans 8:19-25)