Here again we have an unconditional demand of discipleship which serves to remind us of our need for grace. This parable, and the one following, are unique to Luke. Similarly, a disciple should not take the responsibilities of discipleship lightly. Faced with such an enemy a wise person would sue for peace. The parable serves to teach the disciple to consider the cost.
Leeched salt is worthless salt.
The NaCl is leeched out leaving a useless white powder, good for nothing. This saying serves to sum up the previous parables. Jesus concludes with his favourite prompt to hear right. His words are a riddle, and are only for those with ears to hear. Impossible Discipleship Commentators on this passage tend to take one of two lines of interpretation.