In Luke 18, Jesus speaks a parable to His Disciples/future Apostles so that, "men always ought to pray and never lose heart"!
He then tells them of the unjust judge who, "did not fear God nor regard man" and the widow who came in front of him to be avenged, to receive (vindication, retaliation, vengeance or punishment) to the person who was her adversary.
In verse 7 The Lord says this, "And shall God not avenge His own elect who cry out day and night to Him, though He bears long with them? 8 I tell you that He will avenge them suddenly. Nevertheless, when the Son of Man comes, will He really find faith on the earth?"