"If you know yourself then you know the Kingdom of God."

*note* this post was written in some forum,I forgot who wrote it.Anyway,it is interesting.
-added by danny-

"Self-realization, a fact of experience

Enunciation of the nature of the Atman (soul, Self) forms the
central theme of the Bhagavad Gita and the Upanishads. A common
feature of their approach is that this abstract spiritual knowledge
is conveyed through a dialogue between the Guru (preceptor) and the
Sishya (syudent). In the Bhagavad Gita for instance, the teaching
unfolded in the battlefield when Arjuna expressed his despondency to
Lord Krishna. The Lord who had till then been his charioteer assumed
the role of his teacher.

In the second chapter is the Lord's teaching of the nature of the
Atman to convince Arjuna about its eternal nature so that he would
not grieve over death which was inevitable in the war. The Lord
pointed out to him, "There is no permanent existence for the
destructible bodies. The soul, on the contrary never ceases to be."
Only the wise understands the real nature of these two. Understand
that the soul which pervades, penetrates and comprehends is
imperishable and not being liable to destruction, nothing can cause
its destruction."

In his lecture on the Bhagavad Gita Swami Paramarthananda said that
consciousness was intrinsic to the soul and it was external and
could exist independently of matter. This is a very important
feature of Vedanta which holds consciousness as the basis of
existence. This is opposed to the predominant view of science which
postulates matter as the fundamental principle and consciousness as
adventitious to matter. So according to Vedanta matter is only a
temporary phenomena.

Consciousness is always the experiencer and cannot become the object
of experience. Further it is not subject to growth and decay; and is
not a doer (akarta) or an enjoyer (abhokta). So it can never be
touched by Karma. Birth and death which take place according to the
dictates of Karma thus does not afflict the soul.

This is the reason why the Lord has said that it was only a wise
person (Jnani) who could truly know the nature of the Self. What is
the Jnani's understanding? He has the knowledge of the Atman as the
eternal consciousness which is indestructible, changeless, birthless
and decayless. This knowledge takes place in two stages.

When the student learns this from his teacher he first identifies
himself with his mind and body and is under the assumption that he
has an Atman which is eternal, unlike his body. By internalizing
this knowledge through constant meditation the student identifies
himself with the Self and understands that his body is adventitious
to it, which is the result of Self-realization. The whole process is
not just verbalization but a fact of experience." (Pragati, Vol.21,
No.12 - Dec. 2005)