Sage Sankara wanted to establish the existence of the Brahman or God in truth.+

By saying I know the truth; I know Brahman is not wisdom. The Truth is found only in the Unity of understanding which is the knowledge of both the matter and spirit. That is the matter and the spirit is one in essence.  And that essence is the Spirit, which is present in the form of the soul or the consciousness.

When one knows the matter and  the spirit together, one has “Gnana or wisdom." One must have the humility to get to the very root of the matter.

Bhagavad Gita:~ brahmano hi pratisthaham Brahman is considered the all-pervading consciousness which is the basis of all the animate and inanimate entities and material. (Gita 14.27)

The Upanishads say in effect that: ~ If you believe that you are one and God  (Brahman) is another you cannot understand Truth. 

Sage Sankara: ~ Atman, the Self is verily Brahman (God in truth), being equanimous, quiescent, and by nature absolute Existence, Knowledge, and Bliss. Atman is not the body that is non-existence itself. This is called true Knowledge by the wise. 

The word Brahman means ultimate truth or reality which cannot be indicated by any word. The Brahman can be expressed through silence because it is beyond the experience of form, time, and space.  Therefore, the word Brahma clearly stands for the essence of the three states, which is consciousness only. The final use of the pursuit of truth is to know that self is consciousness.

Sage Sankara opposed the Buddhists only, who misunderstood Buddha and became atheists. According to Sage Sankara meditation always means a critical analysis of the Self to get salvation from worldly tensions. Due to the eccentric ego of the then atheists, Sage Sankara did not go beyond this since the atheists will not accept God beyond themselves. This limitation is not due to limited knowledge of Sage  Sankara but is due to the then-existing situation of the psychology of the surrounding society. Even Buddha kept silent about God because the society dealt by Him consisted of Purvamimamsakas, who were strong atheists. Buddha told that everything,  including the Self, is only relatively real (Sunya). This is correct because the self is a part of the universe, which is relatively real with respect to the absolute unimaginable God.  Buddha stopped at this point because the atheists cannot realize the existence of the unimaginable God indicated through His silence. 

The point of Buddha is that if God is non-existent, the entire creation,  including the Self is non-existent. Sage Sankara wanted to establish the existence of the Brahman. For this purpose, He made the Atman as the Brahman. He brought out the identity of self with consciousness and made the Atman the Brahman. Since one will not negate the existence of the Self, he will accept the existence of the Brahman, which is the Atman or soul, the Self. Both Buddha and Sage Sankara kept silent about the absolute unimaginable God. The same philosophy was dealt with by them from different angles in different situations. : ~Santthosh Kumaar