Luke 18:1

And he spoke a parable unto them
To his own disciples, as the Ethiopic version reads, in order to encourage them to prayer, with perseverance in it; since such sore times of trial and affliction were coming upon the Jews, of which he had spoken in the preceding chapter; and such times more especially call for prayer; see ( Psalms 50:15 )

[to this end], that men ought always to pray.
This is opposed to them, who pray not at all, or have left off prayer before God, or who pray only in distress; and suggests, that a man should pray as often as he has an opportunity; should be constant and assiduous at the throne of grace, and continue putting up his requests to God, though he does not presently return an answer:

and not to faint;
by reason of afflictions, temptations, desertions, and delays in answering prayer; and prayer itself is an admirable antidote against fainting under afflictive providences: it is with the Jews an affirmative precept that a man should pray, (Mwy lkb) , "every day" ^F11; it was usual with them to pray three times a day; see ( Psalms 55:17 ) ( Daniel 6:10 ) there is no set time fixed by Christ; men should be always praying. This is not to be understood, that a man should be always actually engaged in the work of prayer; that he should be continually either in his closet, in private devotion to God, or attending exercises of more public prayer, with the saints; for there are other religious exercises to be performed, besides prayer; and besides, there are many civil affairs of life, it is every man's indispensable duty to regard: nor does our Lord mean in the least to break in upon, or interrupt the natural and civil duties of life; but his meaning is, that a man should persevere in prayer, and not leave off, or be dejected, because he has not an immediate answer; and this is clear from the following case.