Explanation:

God needs even the highest sinner as His medium to test the knowledge and faith of a devotee. For example, Shankara was the top most scholar and devotee of God (here the aspect of human incarnation of Shankara is set aside because Shankara acts in the role of a devotee only to preach the other devotees). Shankara can easily recognize the mediated God due to His powerful vision of knowledge. To mislead such a great genius, the medium of a top most sinner is needed for God. God came in the form of a cobbler drinking wine and using foul language. God was associated with the most sinful soul in this gross body of cobbler. The medium was so powerful that even Shankara could not recognize God and was misled. Hence, even the top most sinner has a lucky chance to get the service for God. God created this world with various types of souls for His play only and thus, a soul has no right to criticize another soul for its sins. The world is not created for the purpose of souls, since it is created only for the purpose of the play of God for His entertainment. When a sinner exists for such purpose to be taken as a medium in the play of God, who are you to criticize that sinner? Shankara criticized the cobbler and His mistake was proved. Suppose a plate of meals with different items is served for a king. One of the dishes may be very hot with chilies and king enjoys that dish very well. Mean while, an ant tasted the hot dish and started criticizing the cook for the chilies. The meals and the hot dish are not meant for the petty foolish ant! Similarly, the good and bad represented by the three qualities (Good Sattvam and bad Rajas and Tamas) are not created for the sake of the souls. In fact, the souls are also a part and parcel of the meals to be enjoyed by God. You can assume the king to eat the ants also as dish and such food habit exists in this world in a foreign country! Hence, the soul should be beyond both liking and disliking in this world and should always concentrate about its behavior to suit for the liking of God. Under such back ground, the soul should be neutral to friend and enemy without any personal taste (Suhrunmitraaryudaasinah…. The Gita). The soul should not have any personal liking and must develop the liking of God only as its personal liking. Such stage is the final in the spiritual progress and such soul, which realized the purpose of its creation, is only selected by God for the sake of becoming human incarnation. Nothing is good or bad by itself. Anything is good if it is in the service of God and anything is bad if it is in the service of the world, and this concept is valid only when God competes with the world (Nivrutti). However, within the limits of the world without God, the good and bad have their own distinction based on the divine scriptures of ethics.