Augustine of Hippo — as excerpted in the Catena Aurea on Luke 18:1-8

“The widow may be said to resemble the Church, which appears desolate until the Lord shall come, who now secretly watches over her. But in the following words, And she came unto him, saying, Avenge me, &c. we are told the reason why the elect of God pray that they may be avenged; which we find also said of the martyrs in the Revelations of St. John, (Rev. 6:10.) though at the same time we are very plainly reminded to pray for our enemies and persecutors. This avenging of the righteous then we must understand to be, that the wicked may perish. And they perish in two ways, either by conversion to righteousness, or by punishment having lost the opportunity of conversion. Although, if all men were converted to God, there would still remain the devil to be condemned at the end of the world. And since the righteous are longing for this end to come, they are not unreasonably said to desire vengeance.”