Upanishads are abundant source of knowledge to realize the Brahman.  Isha Upanishad, also known as Ishavasya Upanishad originates from sukla Yajur Veda and is one among the few Upanishads which is very concise and explains the Brahman in a nutshell.  The whole Upanishad is in the form of 18 couplet verses, whereas most of the Upanishads are in prose format. It is said that each of these verses represent one chapter of Bhagavad Gita.  This Upanishad is so called because the first verse begins by saying ‘isha-vasyam-idam sarvam' which means ‘all this covered by the Brahman'.  Since this verse begins with the word ‘Isha' it is called Isha or Ishavasya Upanishad.  Following is the message of this Upanishad in brief.

The universe constantly undergoes change, but the Brahman (Isha) does not change.  Develop non-attachment and awareness of the Supreme Self.  Do not begrudge for others wealth.  For long life, one should perform prescribed duties without attachments and the results arising out of such actions without attachment or desire will not accrue to the one who performs such actions.  There are certain worlds engulfed by darkness (hell) and this is the place where ignorant men (lack of knowledge of the Brahman) who perform acts of iniquity, reach after death.  Brahman is without a second and all pervasive.  Though He is static, He is faster than our thoughts.  When it says static, the Upanishad talks about the Brahman without attributes, the pure form of the Brahman, the original Creator called Hiranayagarbha.  Thoughts and other related activities arise from the Brahman with attributes.  This form of Brahman is called maya.  The pure form of the Brahman is the pure form of consciousness.  He is the creator and sustainer of this universe.  It is not possible to localize Him, as He is omnipresent.  He is movable, yet He does not move.  He is far off, yet He is near.  He is within, yet He is far away.  The one who visualizes Him in everything and everything is visualized in Him, does not hate anything.  When such a person perceives that everything is Brahman as a single entity, he turns beyond bondage and delusion (Krishna calls this man as ‘gunatitah' meaning preternatural to the corporeal modes of nature (Bhagavad Gita XIV.25).  That Brahman is omnipresent, self-illuminating, amorphous, without blemish, pure, impeccant, omniscient, ruler of mind, pervasive, without parentage, eternal and sustains all as per their karmas.  Those who are bound by ignorance and offer sacrifices with attached desires enter into darkness of bondage.  But those who pursue the knowledge of wisdom by worshipping different forms of gods and goddesses enter into greater darkness (Here the Upanishad emphasizes on inner realization).  Wise men say that what is derived from ignorance is different from what is derived from knowledge.  The one who pursues both (rituals) attains immortality.  Those who worship un-manifested forms enter into darkness and those who worship manifested forms enter into deeper darkness (It is about the cause and effect of creation.  The Upanishad says that without realizing the Absolute any method of worship is useless).  The one who worships both un-manifested and manifested at the most attain immortality.  At this point, the Supreme Self or the Brahman is not realized and practitioner continues to be attached only to rituals.

The face of eternal Truth is hidden by sun.  The practitioner addresses the sun by saying "Oh! Sun! The sustainer of this universe, please remove that veil for me so that I can see the eternal Truth.  Oh! Sun, son of Prajapati (Lord Brahma), the unequalled sustainer of the solar system, please disengage your rays by withdrawing light.  I want to see the auspicious form of the Brahman there.  I am That Brahman."  Sun is compared to the Brahman as an example.  Though sun also derives its light from the self-illuminating Brahman, but in order to make us understand the underlying principle, sun's illumination is taken as an example.  Finally, the practitioner understands the principles of self-realisation ‘I am That (Brahman).'  Now the practitioner faces death and addresses god Agni thus:  "Let my breath be merged with the eternal element of air.  Let my body consigned to fire be reduced to ashes.  Oh! God in the form of OM, in the form of fire, think repeatedly about me and karmas done by me."  The Upanishad ends with a universal prayer to lord Agni.  "Lead us in the righteous path.  Lord! You are aware of our thoughts.  Please destroy all our invisible evil thoughts.  We offer our prayers to you again and again."  This is the prayer seeking no rebirth, a phenomenon called liberation.