My favourite Upanishad is the Isha Upanishad ....

In India they call jesus Isha and the greatest Upanishad ever written is called the Isha Upanishad. It goes a little something like this.....

Peace Chant

OM! That (the Invisible–Absolute) is whole; whole is this (the visible phenomenal); from the Invisible Whole comes forth the visible whole. Though the visible whole has come out from that Invisible Whole, yet the Whole remains unaltered. OM! PEACE! PEACE! PEACE!

The indefinite term “That” is used in the Upanishads to designate the Invisible–Absolute, because no word or name can fully define It. A finite object, like a table or a tree, can be defined; but God, who is infinite and unbounded, cannot be expressed by finite language. Therefore the Rishis or Divine Seers, desirous not to limit the Unlimited, chose the indefinite term “That” to designate the Absolute. In the light of true wisdom the phenomenal and the Absolute are inseparable. All existence is in the Absolute; and whatever exists, must exist in It; hence all manifestation is merely a modification of the One Supreme Whole, and neither increases nor diminishes It. The Whole therefore remains unaltered.

I - All this, whatsoever exists in the universe, should be covered by the Lord. Having renounced (the unreal), enjoy (the Real). Do not covet the wealth of any man. We cover all things with the Lord by perceiving the Divine Presence everywhere. When the consciousness is firmly fixed in God, the conception of diversity naturally drops away; because the One Cosmic Existence shines through all things. As we gain the light of wisdom, we cease to cling to the unrealities of this world and we find all our joy in the realm of Reality.

More .... http://www.yoga-age.com/upanishads/isha.html