John Lightfoot on Luke 18:1

[And not to faint.] The discourse is continued still; and this parable hath its connexion with chapter 17, concerning Christ's coming to avenge himself upon Jerusalem; which if we keep our eye upon, it may help us to an easier understanding of some more obscure passages that occur in the application of this parable. And to this doth the expression not to faint, seem to have relation; viz. that they might not suffer their hopes and courage to languish and droop, upon the prospect of some afflictions they were likely to grapple with, but that they would give themselves to continual prayer.