When serious seeker reaches a stage with all his baggage of accumulate knowledge, then uncertainty haunts him

Doubts and Confusions arise when seeker finds the scriptures and yoga are inadequate or useless to quench his spiritual thirst. Disappointments in religion or yoga or even science imply error or ignorance.

When serious seeker reaches a stage with all his baggage of accumulate knowledge, then uncertainty haunts him. Whether he is right?" Where is the certainty that he is proceeding on right lines?" Thus doubts arise and the inquiring spirit comes and impels to search elsewhere for truth where it will not be possible even to have doubt. The test is therefore in experience. And only in non-duality, where there are no two to argue about views or to have difference of opinion can such doubtlessness be possible. Belief depends upon unstable bases whereas certainty depends on proof.

Seeker of truth has to get rid of his doubts through deeper inquiry, analysis and reasoning on his own, to realize the fact that, the self is not the form but self is formless. Thus his analysis and reasoning has to be based on the formless not on the form. By simply go on believing and accepting whatever said by the punditry will not lead one towards path of wisdom. All the doubt has to be got rid of "by the sword of Self - Knowledge." The scriptures, yoga are not necessary if one follows the inner (formless) path. Religion and yoga are not the means to path of wisdom.

They alone in this world are endowed with the highest wisdom who are firm in their conviction of the sameness and birthlessness of Atman. The ordinary man does not understand their way. [Chapter IV — Alatasanti Prakarana 95-P-188 in Upanishads by Nikilanada]

This passage indicates the fact that everything is soul or consciousness  or spirit. The one who views and judges the worldview on the base of his birth  or body will not realize the fact that, the man including the world is created sustained and finally dissolves as soul or conciousness. Thus Atman alone is, and all else is mirage.

Thus Self-Knowledge or Brahma Gnana or Atma Gnana  is the aim of every human being. Since everyone thinks the physical body is the self, their aim is misdirected and they focus their attention on materiality which makes one feel the duality (waking) as reality.