St. Luke uses the concept of prayer to encourage his community not to lose heart. The people of  his day had their challenges; they had experiences of evil, not unlike what we have today.  Persistence is a sign of trusting in God's providence, especially the longer we wait for his response. However, the experience of waiting is often the way in which we learn God's response, namely, his will. This experience of waiting also keeps us faithful as we await the final coming of  the Lord.