Jesus compares God, not with a good man, but with a godless man to emphasize the vast difference between this unjust judge and the righteous God. The conduct of the unjust judge exposes the chaotic and corrupt judgments in which he had prostituted himself. No one can compel him to do anything because he feels no regard for anyone, including God. He acts purely out of self-interest. Yet, if this unjust judge could avenge a widow whom he disdained, how much more will the righteous God avenge his elect (Jeremiah 11:20)?
This parable reveals God's willingness to hear and answer the supplications of His elect (Luke 18:7), responding when they are offered according to His will. The word "avenge" (verses 5, 7-8) implies the working out of His vengeance in justice, not in retaliation. If God's elect are wrongly treated, they can be sure of vindication. So, if the unjust judge because of a selfish irritation avenges a troublesome widow, how much more will the selfless God come to our aid? We can expect substantially better treatment from a God of lovingkindness than from a heartless judge.
The widow, who speaks only five words, does not prevail because of her persuasive plea but because of her persistence. Sometimes too many words reveal a scarcity of desire or a lack of purpose. Jesus tells us long prayers and useless repetitions will not make God hear us any better (Matthew 6:7). He already knows our needs (verse 8).
God has assured us that He hears and answers prayer. We must have the faith of Christ that God can provide what we need, enjoys hearing us ask according to His will, and desires to give us abundantly what we should have.