Sage Sankara pointed out; even those who were outside the Upanishad fold were as eligible to moksha.+

Ashtavakra:~There is no wisdom whatsoever in the scriptures-just a collection of words.

The seeker should not bother about finding the meaning of what is written in the scriptures. To argue with a person who has renounced the use of reason is like injecting medicine into a corpse.

 Mediocre minds waste their time arguing about unimportant matters. Accumulation of scriptural knowledge is dangerous in pursuit of truth. The seeker has to establish the truth of Non-duality by sheer reasoning alone. 

The seeker has to begin by defining "What is real?" "What is unreal?" etc, because that is the right way to discuss, understand and assimilate Self-knowledge. Thus seeker has to remove all the hindrances to acquire Self-knowledge.

In dualistic philosophies, the imaginary god is made as a creator basing on cause and effect theory. Then the doubt natural arises, who is the cause of the creator. The creator is dependent on creation for his existence. 

Without creation, the creator ceases to exist. Thus the dualistic theory holds no water because it is based on mere imagination and speculation, which they try to prove on citing the mythological stories and doctrine of code of conducts. 

When the Self is not the body then all these dualistic ideologies hold no water. Therefore the devotional paths, which are based on human conduct, are not the means for truth -realzation. 

Bhagavad Gita 2:46:~ "A man of true knowledge who has attained enlightenment, has the same use for all the scriptures as one has for a small reservoir of water in a place flooded on all sides."

Sage Sankara strongly advocated the study of Upanishads, and at the same time cautioned that the study of Upanishad alone would not lead to moksha. In matters of such as spiritual attainment, one’s own realization was the sole authority and it cannot be disputed

Sage Sankara also said the study of Upanishad was neither indispensable nor a necessary prerequisite for attaining the human goal, the moksha.

Sage Sankara pointed out; even those who were outside the Upanishad fold were as eligible to moksha as those within the fold were. He declared that all beings are Brahman, and therefore the question of discrimination did not arise. All that one was required to do was to get rid of ignorance (Avidya or duality).

The seeker's aim is the search for the Ultimate Truth or Brahman. The search to find the non-dualistic or Advaitic truth that in actuality never was lost, only hidden.

Upanishad aspiration is best expressed in the following sutra:-

OM Asato ma sad gaMaya , tamaso ma jyotir gaMaya , mrityor ma aamritaam gaMaya . Shanti, Shanti, Shanti

"OM Lead me from ignorance to truth, from darkness to light, from death to immortality. Peace, Peace, Peace" (Brihadaranyaka Upanishad (1/3/28).

Upanishads are based on the insights of the sages and seers, serves primarily as a guidebook. One has to accept the Ultimate reality as Brahman or God in truth.

The Sruti itself’ says:~ "This Atma is NOT to be attained by a study of the Vedas. (Katha Upanishad I, 2, 23.)

Sage Sankara:~ VC- 162- There is no liberation for a person of mere book-knowledge, howsoever well-read in the philosophy of Vedanta, so long as one does not give up the false identification with the body, sense organs, etc., which are unreal.: ~ Santthosh Kumaar