Universe of course is illusory, but there is Brahman, that exists forming the very substratum of all things.

Universe will not remain as reality when wisdom dawns. Universe is mere mirage created out of consciousness and there is unity in diversity because there is no second thing exist other than consciousness.

One thinks he is imprisoned within this body; whereas the body and his experience of the world are within the mind. The consciousness is hidden within the three states but it is without the three states.  The one, which has the awareness of the three states, is not the body but the soul, which is in the form of consciousness. Thus consciousness is not limited to waking experience alone because it pervades all the three states. until one views and judges the world-view on base of ego he is in the grip of individuality or intellectuality.  The self is not an individual because it is universal. And it pervades in everything and everywhere in all the three states. Thus individualized judgment will not lead one non-dual destination.  

 That is why Sri,Sankara says in Viveka Chudamani :-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.

thus one has to know what is real by realizing our body and our experience of the world is mere illusion created out of consciousness, which the inner most self.  The nature of the self is emptiness. And it is identified by different masters with different name, such as Brahman, Buddha’s nature, Christ consciousness, Self, Ultimate Truth.

Buddhism says: all things are illusory and noting exists.  However, Advaita avers that it is not so.  It says that the Universe of course is illusory, but there is Brahman, that exists forming the very substratum of all things. 

Self-Knowledge or Brahma Gnana or Atma Gnana is necessary to realize the ultimate truth. Self –Knowledge or Brahma Gnana it is only for those who has intense urge to know the ultimate truth or Brahman or Inner Buddha or Christ.