If he remains inactive like yogi in a cave can never realize the ultimate truth or Brahman or God in truth.+

Sage Sankara himself had often said that his philosophy was based on Sruti, or revealed scripture.  This may be because Sage Sankara addressed the ordinary man, who finds security in the idea of causality and thus, in the idea of God—and Revelation is indispensable to prove the latter.  He believed that those of superior intelligence, have no need for this idea of divine causality, and can, therefore, dispense with Sruti and arrive at the truth of Non-Dualism by pure reason. 

Only through Soulcentric reasoning, one will be able to cross the domain of form, time, and space.

Ashtavakra Gita: ~ It is not the absence of Buddhi (reason) that can grasp Advaita but the man who possesses the highest intellect. Brains are necessary. Such a man, by merely hearing the truth mentioned will know it. (Page~ 224).

The one who possesses the highest intellect can grasp the Advaitic truth, by merely hearing the truth mentioned will know it.

Bhagavad Gita: ~ Reason as a means to reality. (Chap.18 verse 37)

In chap. 10 of in Bhagavad Gita, Krishna says ~ I cannot save you, but I can give you Buddhi (reason). (chap. 10)

In chap. 10 of in Bhagavad Gita, Krishna says: ~ "I have given you the most secret teaching, now reflect over it all" Krishna plainly says reflect, think. (Verse 63 of Chap. 18)

Mundaka Upanishad says: ~  Both states are harmful and take you away from the path of inquiry into truth. (Page 234)

The reasoning is interpretable in two ways. The egocentric interpretation is to apply it only to the practical purpose (waking experience). The Soulcentric interpretation is to apply it to the three states. The latter leads to a final settlement of the problems because it takes all data into consideration.

If he remains inactive like a yogi in a cave can never realize the ultimate truth of Brahman.

The true meaning of "kill doubt" is not to refrain from inquiry as Pundits, yogis and religionists say, but to tackle every doubt and to go on until you answer or solve it satisfactorily and thus the doubt disappears.

Lord Krishna tells Arjuna to fight is misrepresented by half-Vedantins as an order to kill other human beings, because they are mere Ideas, Illusory, whereas whole Vedanta says these ideas too are Brahman, and the Self and hence no killing really occurs. Only when you see all individuals, especially the Self as imagined ideas, can you rise to see them later as Brahman? Thus, there are two stages. You must first see the ‘I’ as illusory before you see others as illusory. ~ CH.2 v.16

Bhagavad Gita gives dualistic worship of "God” only for the lower minds; it also teaches Advaita for the more evolved.

Likewise, thinkers and poets of the Age of Devotion (Bhakti) of the 16th century believed in a God with attributes who became very tangible when incarnating as Avatar and was attainable simply through love and devotion rather than scholastic and intellectual meditation.

For the religious people, the Bhagavad Gita became the main vehicle of inspiration with its qualified and deistic Monism, rather than the scholastic and esoteric path shown by Sage  Sankara’s Advaitic path.

Devotional prayer (Bhakti) is valued only for the ignorant who believe the world in which they are born is real. But for the seeker on his journey to the ultimate realization of the truth beyond the form, time, and space, the devotional path becomes an obstacle.

People worship God in various ways, not knowing the Truth. At different levels, at different epochs, in different lands, people have different conceptions of God. They have quarreled because they did not know the truth about God.

The conflict of opinions among mystics and religionists proves that all are imagining God as they like, not knowing God.

Rig Veda: ~ The Atman (Soul or Spirit) is the cause; Atman is the support of all that exists in this universe. May ye never turn away from the Atman, the Self. May ye never accept another God in place of the Atman nor worship other than the Atman?" (10:48, 5)

Rig-Veda 1-164-46 and Y.V 32-1 clearly mention that God is “One”.

Rig Veda declares God is ‘ONE’ and God is Atman, then why believe and worship in place of real God.

Brihad Upanishad: ~ “If you think there is another entity, whether man or God there is no truth."

When Upanishad itself declares: ~   Sarvam khalvidam brahma ~ all this (universe) is verily Brahman. By following back all of the relative appearances in the world, we eventually return to that from which it is all manifest – the non-dual reality (Chandogya Upanishad). 

Sage Sankara’s Supreme Brahman (God) is impersonal, Nirguna (without Gunas or attributes), Nirakara (formless), Nirvisesha (without special characteristics), immutable, eternal, and Akarta (non-agent). It is above all needs and desires. It is always the Witnessing Subject. It can never become an object as it is beyond the reach of the senses. Brahman is non-dual, one without a second. It has no other beside it. It is destitute of difference, either external or internal. Brahman cannot be described because the description implies a distinction. Brahman cannot be distinguished from any other than It. In Brahman, there is not a distinction between substance and attribute. Sat-Chit-Ananda constitutes the very essence or Svarupa of Brahman and not just Its attributes. The Nirguna Brahman of Sage Sankara is impersonal.

Sage Sankara: ~"That which permeates all, which nothing transcends and which, like the universal space around us, fills everything completely from within and without, that Supreme non-dual Brahman  (God in truth)."

Thus, truth realization is Self-realization. Self-realization is God-realization.  God-realization itself is real worship.  :~Santthosh Kumaar