In the previous chapter, Jesus spoke about the kingdom of God (Luke 17:20–37). Its coming will take longer than many expect. For that reason patience and vigilance will be needed. Now Jesus tells a parable where he urges us to continue praying during this time of waiting. Persevere in your speaking to God! Jesus has often commended his disciples to do so (see Luke 11:1–13). Moreover, persevering in prayer is mentioned surprisingly often in Paul's letters (e.g., Romans 12:12; 1 Thessalonians 5:17). Apparently, we desperately need this encouragement. As humans we are quickly grow weary in prayer when the wait is long. Through a parable, Jesus once again wants to clearly emphasize to us that we must always keep praying.
1 And he told them a parable to the effect that they ought always to pray and not lose heart.