Please read Persistence in Prayer, by Jack Kelley, at Grace Thru Faith.
"Luke 18:1 tells us Jesus gave His disciples this parable to encourage them to always pray and not give up. I think His point in the parable was that if even a corrupt judge will eventually respond to persistence, how much more will the Lord do so. His point that persistence is a demonstration of faith, not the lack thereof, was made in his final comment, “However, when the Son of Man comes will He find faith on the earth?” (Luke 18:8)"
Now He was telling them a parable to show that at all times they ought to pray and not to lose heart, saying, "In a certain city there was a judge who did not fear God and did not respect man. "There was a widow in that city, and she kept coming to him, saying, 'Give me legal protection from my opponent.' "For a while he was unwilling; but afterward he said to himself, 'Even though I do not fear God nor respect man, yet because this widow bothers me, I will give her legal protection, otherwise by continually coming she will wear me out.' "And the Lord said, "Hear what the unrighteous judge said; now, will not God bring about justice for His elect who cry to Him day and night, and will He delay long over them? "I tell you that He will bring about justice for them quickly. However, when the Son of Man comes, will He find faith on the earth?"