If he gives up the external world in his inquiry, he cannot get the whole truth.

In the realm of truth, the belief system has no place. The birth, life and death itself is mere illusion on the ultimate standpoint.  Therefore, the reincarnation, resurrection and rebirth theories are mere intellectual speculation.  For one who is aware of the fact that, the self is not physical but formless Spirit, for him   the question of being Christen, Hindu, Jew or Muslim does not arise.

The man who is aware of the truth knows the intellectuality is mere speculation or imagination  based on the  physical self(ego)  within the illusion. 

Till one remains stagnant with his accumulated knowledge as truth, he will not be ready to verify anything other than his accepted truth, because he is in the clutches of his  egocentric knowledge.  

Meher Baba said: - Give no importance to creed, dogma, caste or the performance of religious ceremonies and rites. 

Jesus said, "Know what is in front of your face, and what is hidden from you will be disclosed to you.

It means one has to know the world which confronts him confronts to realize his body and world are made of the same essence and that essence is Spirit([father). 

Sri, Sankara says:- one must first know what is before him. If he cannot know that, what else can he know or understand? If he gives up the external world in his inquiry, he cannot get the whole truth.

VC- 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.

Jesus said, "Those who seek should not stop seeking until they find. When they find, they will be disturbed. When they are disturbed, they will marvel, and will reign over all. (And after they have reigned they will rest.)"

When the seeker indulges in deeper self-search becomes aware of the fact that, the experience of birth, life, death and the world are mere illusion created out of spirit, he  will be in fear of losing their physical identity  and the experience of the world which  he dearly attached.   But gradually his conviction grows about the innermost self the mid becomes still and rests in consciousness, the innermost self, which is god or Christ.