An interesting parable along with it's conclusion being so apropos for Christians worldwide. There is more than on lesson for us here, the first being that of persistent prayer. Surely our Lord hears our every cry. The Lord emphasized persistence in another parable in Luke 11:5-13. There is an interesting and somewhat puzzling conclusion to that parable when Christ said: "...how much more will your heavenly Father give the Holy Spirit to those that ask Him." But wait! Doesn't the Holy Spirit already reside within every believer? Yes, He does, so the meaning of this can't be a second helping of 'indwelling', rather it is the Holy Spirit's involvement in the answer to our persistent prayer. Regrettably, we seldom recognize His involvement due to our ignorance or even an unknown coldness of heart. Despite our struggles with 'little faith' our Lord does not short-change us, not ever. He always gives what we need---need, not want. The persistence in prayer is not for His benefit, nor is it the motivating factor in His answer. Persistence in prayer is a blessing to us, for God knows what we need before we ask for it (Matt. 6:32).
The Luke 18 parable takes a different turn. It goes farther than the meeting of our daily needs in that it deals more specifically with persecution. The avenging of those persecuted and/or killed for the name of the Lord Jesus is a serious matter to Him. Though it may seem sometimes that He has forgotten their end, He has not but will exact proper and righteous judgment in a timely manner (albeit not soon enough for some). Rom. 12:19; Deut. 32:35; and Heb. 10:30 leave no doubt in this matter. "All who desire to live godly in Christ Jesus will suffer persecution" 2 Tim. 2:12. This does not necessarily mean that you have to be physically threatened, or be killed for His name to say that you have been persecuted. No doubt many face death for His name on a daily basis, but not so much in the "West"---at least not yet. Our flavor of persecution is mostly mental and psychological lead by an ever increasing pack of wolves who desire nothing less than your capitulation to any and all of their false doctrines they throw at you. They will attack the Word of God, it's veracity, it's sufficiency, it's perfection, it's immutability, and it's survival since it was written as the still inspired Word of God. The endless parade of "revised editions" haunts those who have accepted them as legitimate while the false, heretical doctrines that emanate from these ungodly revisions have plagued the church for decades. The tentacles of ecumenism are found everywhere and the "unity" song is sung louder day by day. Acceptance of all, no matter their beliefs, is the new mantra. You don't even have to slap a "Christian" sticker on your back any more, a simple belief in an unspecified god will open all the religious doors of our day. Pastors are little more than hirelings, doing the bidding of anyone except the Lord Jesus. They will accumulate "dossiers" on the members of their church who dare to question the leadership. They will seek to enlist close family members to sway an objector to their way of (evil) thinking all the while using flattery to gain advantage by lowering your instinctive defensive mechanisms. All of this and more will assault the believer of today and this is where the persistence of prayer comes in. For some it becomes more than a few dozen times a day that they pray, but 24/7. You may seem like you eat it, drink it, sleep it, dream it for days or weeks on end, but so have our brethren of past days (Heb. 11; esp. vs. 30-40). Then there is 1 Peter 5:6 & 7; "... cast all your care upon Him, for He cares for you." Your persistent prayer will be answered with abundant grace, you will not only survive the ordeal, but rejoice in the fact that you went through in the first place (1Peter 4:12 & 13 and 1Peter 1:3-9). He will ALWAYS keep you, He will ALWAYS answer your prayers, and live or die you are ALWAYS His. Rev. 6:9-11.