The Upanishads constitute the central basis of Hindu Religion and philosophy .They are the Vedanta or the end of the Vedas, the culmination of Knowledge.Nothing can be a match to the wondrous suggestiveness of the Upanishads. They have satisfied the greatest thinkers of the world, and they have pacified the greatest spiritual men here. Nothing that went before or after has been able to surpass the Upanishads in the depth of Wisdom and the message of Satisfaction and Peace. Dadhyanch ,Uddalaka, Sanatkumara ,Sandilya and Yagnavalkya are some of the outstanding philosophers and Sages of the Upanishads who have lit up the torch to the path of perfection. The Upanishads mainly preach knowledge through philosophising. They are the text-book for the seeker after the Self. They are styled by different names ,Brahma Vidya ,Adhyatma –Shastra ,Vedanta ,Jnana.
One who practices teachings of the Upanishads attains to the supreme. He breaks the knot of the heart, clears all doubts and destroys all sins. He enters into the All. He is liberated from embodiment .He becomes immortal .He becomes the Self of all. He is an Apta Kama. He is really blessed. He crosses over sorrow. He crosses over sin. He does not return to the mortal coil .He exists as the absolute.
The Upanishads are a book of spiritual knowledge. The Supreme is pervading all that appears here. One should therefore really enjoy by renouncing the sense of separateness .He has no reason to covet other’s property.
Life is not a misery. One should live for hundred years by performing action without attachment. Life is not a bondage when it is looked with proper light. Such a man of proper knowledge looks on all beings as his own Self and his Self as all beings. To him everything is his own Self and he is not affected by grief ,delusion or sorrow of any kind.
(Source : A book called Upanishad Sara published in 1947.The contents are completely from that book )