Luke 18 records the parable of the unjust judge. If an unjust judge can be convinced to answer a persistent request to avenge a petitioner’s enemies, how much more can God the Father be counted on to avenge his elect?
There are several points to make out of this:
1) I don’t think the parable is talking about being annoying in your prayers, always asking the same thing until God grows tired of you and does what you say. The context is vengeance and God avenging the wrongs done to His children.
2) The Bible makes it clear that vengeance belongs to God and He will fulfill His promise to avenge His people. This should allow us to take the wrong and let God sort it out rather than making us belligerent people fighting for our rights.
3) God will avenge His elect, His bigger question is “when the Son of man cometh, shall he find faith on the earth?” God will avenge His elect, He’s just wondering if there will be any! This shows that election has something to do with the faithfulness of the people and not merely God’s forcing election on unwilling participants. Even He doesn’t know how many the elect will be.
Whether there is faith on earth is up to mankind. We are the ones who resist the Spirit’s work to convict the world of sin, righteousness and judgment. We are the ones who reject so great salvation. Your unbelief is not God’s fault.
God is then faithful to reward and avenge all those who seek Him and come to Him by faith. He is a good God. His goodness leads men to repentance. Unfortunately, people would rather have the world’s goodness than God’s, so they will refuse to turn from their sin.
God wants to avenge, but He only avenges those with faith. You do not want to be on the wrong side of His vengeance.