No English translation of Luke 18:1 adequately captures the sense of μὴ ἐγκακεῖν, "don't be from evil".
Some examples:
"And he spake a parable unto them to this end, that men ought always to pray, and not to faint;" (KJV)
"Then Jesus told his disciples a parable to show them that they should always pray and not give up." (NIV)
"And he told them a parable, to the effect that they ought always to pray and not lose heart." (RSV)
"Now He was telling them a parable to show that at all times they ought to pray and not to lose heart," (NASB)
"Then Jesus told them a parable to show them they should always pray and not lose heart." (NET).
These failures of translation are not surprising given the thoroughgoing effort to suppress the failed eschatological message of Jesus and reinterpret it from the beginning. As usual, however, Luke remains its rare honest reporter.
The translations suffer from reading into Luke's usage of the term, the only one in the gospels, the usage from Paul, which has already become patently psychological and introspective (e.g. 2 Cor.4:16). Luke's use, however, is plainly eschatological in its context (Luke 17:20ff. through Luke 18:8) and knows nothing of this later "introspective conscience of the West". It focuses on the behavior which springs from the inner man, not on the inner man itself. Yes, Scripture ought to interpret Scripture as the Lutherans insist, but it is Luke who ought to interpret Luke.
The one who gives up praying to God in Luke is representative of the many faithless at the end of the world, who are literally "from evil" (ἐκ κακός) because they have given up believing in the very idea of justice in the first place. The many are all jaded and don't even bother asking for justice anymore. In fact, to them the seat of justice itself is so unhinged the effort would be doomed from the start. The representatives of justice have become such thoroughgoing individualists and laws unto themselves who do not see themselves as beings in relation to God or even to other men that it would be impossible even to make a case to them. So why even try?
The few who will be saved, however, are like the persistent widow of this narrative. She alone among all her peers has not given up on the idea of a justice which is outside herself and represents the ground of being. No one else but she even bothers to try anymore. No one else but she even believes that a decent case can be made for it. She is ridiculously outnumbered. The capriciousness of unjust justice she faces at the fullness of time, at the end of the world, is shown in that it is moved no longer by principles of God or man but only by its own exhaustion with this harpy. This lone defender of Absolutes wins because she is stronger and more enduring because of the Absolutes, not because of her faith in the Absolutes. She simply knows the strength of her case, and refuses to give it up. She knows it can't be beaten, and that it will win. That Jesus must admonish even his own closest disciples to be like her and not join the many in their backsliding behavior is very telling. His promises of the imminent consummation were beginning to ring hollow even in their ears.
It calls to mind Jesus' instruction to his disciples elsewhere about the paradigmatic discipleship of a widow, who put into the treasury (ὅλον τὸν βίον αὐτῆς) "her whole life" (Mark 12:44/Luke 21:4), perhaps the most important two cents in the history of the West. For whatever else might be said about the failure of the kingdom of God to appear, Christendom yet stands for that same transcendent, unshakeable moral order for which a widow sacrificed everything that she had.
The human capacity for and ubiquity of evil were taken for granted by Jesus. What remains remarkable is that he believed some could repent, and no longer "be bad".
