Explanation:

The Yoga in Pravrutti (Atmayoga) is totally different from the Yoga in Nivrutti (Brahmayoga) since atheists do not believe in God. The Yoga of Nivrutti is irrelevant to them. The only possible advice to help them is the Yoga in Pravrutti which is the attainment of the stored inert energy or self. The Yoga in Pravrutti is common to both atheists and theists, which is the attainment of perfect physical and mental health that gives happiness, but not the bliss of God.

 This happiness is non-eternal, where as the happiness from God is eternal and is called as bliss. The happiness in Pravrutti yoga is nothing but peace attained by arresting the loss of inert energy in the worldly affairs. Hence, the storage of inert energy is mainly based on the discipline of life and the control of desires, which are responsible for extra activities leading to extra expenditure of inert energy. Since the inert energy cannot be supplied infinitely, by taking infinite food, the control on expenditure is essential. Since the expenditure is only work (Karma) and the initiator of work is the desire or qulaity, control of desires has to be advised in the Yoga of Pravrutti. Hence Buddha stressed on the control of desires, to attain happiness in the case of atheists and this is the reason for His silence about God.

Shankara was also concentrating on atheists and hence had to follow the path of Buddha by saying that God does not exist separately other than the self. This is also another style of atheism. Hence, Shankara was said to be a follower of Buddha in disguise (prachchhanna bauddha). You must remember that neither Buddha nor Shankara were atheists but they had to act like atheists in dealing with atheists. Buddha kept silent about God in the case of atheists. Shankara, being the top most genius, introduced the word Brahman which means both inert cosmic energy as well as God.

Since, self is a part of the cosmic energy and qualitatively the same, Shankara extended the word Brahman meaning cosmic energy to soul also. The word Brahman can be taken as God also in the sense that it is the ultimate. Since, cosmic energy is the ultimate for atheists, it can be called as God of atheists.