Luke 18:12

“I fast twice in the week, I give tithes of all that I get.”

It was customary among the Pharisees to tithe even the smallest thing that they received ‘from God', even when it was not required by the Law (Luke 11:42; Matthew 23:23). Furthermore they fasted every Monday and Thursday, as well as on special days. The purpose of this latter was in order to make them humble, but always the danger was, as in this example, that it could make them inordinately proud (compare Matthew 6:1; Matthew 6:16). Not all prayer is holy.

So all in all God obtained from his prayers a good picture of his pride, his self-conceit and his total self-righteousness. He had justified himself to his own satisfaction, but had revealed all too much to God. For God, who looked at his heart and could only condemn him for the sin that He found within it, would mark him off as another failure.