In two parables, one right after the other, Jesus spoke about prayer in Luke 18. He first gave the parable of the importunate widow seeking justice from the judge and then He gave the parable of the Pharisee and the tax collector with the tax collector crying out for God's mercy. In both instances the Spirit of God led Luke to say that Jesus responded to both prayers with, "I tell you..." What was Jesus "telling" us? He was showing us that God was well aware of the prayers these two very needy people prayed and that their prayers would not go unheeded. The judge of the first parable eventually gave in to the widow's repeated persistency to get justice. The second parable made it clear that the tax collector needed only to cry out for mercy once and his prayer was heard and answered immediately.
I see two good lessons here: First, the Lord wants us to persist in our prayers. He may delay His answer and in some instances He may not do what we wanted, but He will always do what is best. But He wants us to repeatedly cry out to Him while we're waiting for His response. Second, the Lord is very quick to respond to those broken sinners who seek His mercy and don't depend on their own righteousness. The sinner's prayer for salvation is never missed by God and He will justify the repentant sinner's plea for mercy. All of us who are Christians had the same answer given to us when we cried out for Christ to save our souls.
Assurance of God's answers is what Jesus wanted His disciples to have. He wanted them to learn to be importunate themselves, but He also wanted them to know that when someone calls out to God for salvation He will save them. And that means we are to assure them that He did. Are you persistent in your prayers for that which you believe God wants you to have? Do you believe God isn't ignoring you when He wants you to wait? Pray with confidence and pray for others to believe God as well and encourage them to be assured that God listens when they pray.