‘Because for this purpose also I wrote, that I might know the proof of you, whether you are obedient in all things.'
He assures them that the main reason that he had written the severe letter to them was not in order to obtain punishment for the man, but so that he could test out their own obedience to him as an Apostle, ‘in all things', not just what they chose to accept. That was what really mattered. He was not out for revenge. Rather he had wanted to find out if they would again respond to his authority and follow his instructions about everything he had written. And it had turned out well.