Day and night

Luke himself says why Jesus told this story: that men ought always to pray, and not to faint. A widow kept knocking at the door of a judge who cared nothing for God or people. Avenge here means: see justice done.

In the end he granted her request, just to be rid of her. Again a comparison from small to great. If even that judge listens, how much more God, who isn't indifferent but loves.