Talk as much philosophy as you like, worship as many gods as you please, observe ceremonies and sing devotional hymns, but liberation will never come, even after a hundred aeons, without realizing the Oneness.

There are only two mistakes one can make along the road to truth:
not going all the way... and not starting.
— Buddha

Sri, Sankara says : - Talk as much philosophy as you like, worship as many gods as you please, observe ceremonies and sing devotional hymns, but liberation will never come, even after a hundred aeons, without realizing the Oneness.

Thus when we identity with our inherited belief system it with diverse belief than  it becomes difficult to know the whole humanity is created out of single stuff, which is consciousness. The consciousness is identified by different belief system with different names and forms. Until one goes deeper in search it becomes difficult for anyone to accept anything other than his inherited belief.  

Thus following any belief system will not lead one to the inner reality. Thus it is necessary to know the ultimate truth, and the truth will set one free from experiencing the duality as reality."

 That is Sri Sankara, in Bhaja Govindam says: - [Jnana Viheena Sarva Mathena Bajathi na Muktim janma Shatena] - one without knowledge does not obtain liberation even in a hundred births, no matter which religious faith he follows.

Thus it proves that wisdom is universal irrespective of any religion of faith one belongs.  Thus religion is not the means to self-knowledge.  Thus Sri, Sankara’s Advaita minus orthodoxy is true Advaita. 

 Sri, Sankara:- “Illness is not cured by saying the word 'medicine,' but by taking medicine. Enlightenment is not achieved by repeating the word 'God' but by directly experiencing God. Talk as much philosophy as you like, worship as many gods as you please, observe as many ceremonies and sing devotional hymns, but liberation will never come, even after a hundred eons, without realizing Oneness, Wholeness."

Yoga Vasishtha: - "Teachers, interpretations of sacred texts, the force of religious merit--none of these lead to the realization of that Ultimate Truth which is revealed in the clear reflection of the heart, engendered from contact with the good."

Sri, Sankara says in VivekaChudamani:-   

56 . Neither by Yoga, nor by Sankhya, nor by work, nor by learning, but by the realisation of one's identity with Brahman is Liberation possible, and by no other means. 

58. Loud speech consisting of a shower of words, the skill in expounding the Scriptures, and likewise erudition - these merely bring on a little personal enjoyment to the scholar, but are no good for Liberation.

59. The study of the Scriptures is useless so long as the highest Truth is unknown, and it is equally useless when the highest Truth has already been known.

60. The Scriptures consisting of many words are a dense forest which merely causes the mind to ramble. Hence men of wisdom should earnestly set about knowing the true nature of the Self.

61. For one who has been bitten by the serpent of Ignorance, the only remedy is the knowledge of Brahman. Of what avail are the Vedas and (other) Scriptures, Mantras (sacred formulae) and medicines to such a one?

62. A disease does not leave off if one simply utter the name of the medicine, without taking it; (similarly) without direct realisation one cannot be liberated by the mere utterance of the word Brahman.

63. Without causing the objective universe to vanish and without knowing the truth of the Self, how is one to achieve Liberation by the mere utterance of the word Brahman ? — It would result merely in an effort of speech.

64. Without killing one’s enemies, and possessing oneself of the splendour of the entire surrounding region, one cannot claim to be an emperor by merely saying, ‘I am an emperor’.

65. As a treasure hidden underground requires (for its extraction) competent instruction, excavation, the removal of stones and other such things lying above it and (finally) grasping, but never comes out by being (merely) called out by name, so the transparent Truth of the self, which is hidden by Maya and its effects, is to be attained through the instructions of a knower of Brahman, followed by reflection, meditation and so forth, but not through perverted arguments.

66. Therefore the wise should, as in the case of disease and the like, personally strive by all the means in their power to be free from the bondage of repeated births and deaths.

67. The question that thou hast asked today is excellent, approved by those versed in the Scriptures, aphoristic, pregnant with meaning and fit to be known by the seekers after Liberation.

Seeker of truth has to take the direct path and avoid losing precious time and effort loosing himself in philosophical studies.

Fools dwelling in darkness, but thinking themselves wise and erudite, go round and round, by various tortuous paths, like the blind led by the blind. [Ch II-5 P-14]

Seeker of truth has to avoid getting involved with the guru or teaching if wants acquire Self-Knowledge or Brahma Gnana or Atma Gnana  because emotionally getting involved with guru or teaching becomes great hindrance and one will permanently remain in the prison of physicality or duality.