The Isha Upanishad embodies in its very opening verse the central theme of all the Upanishads, namely, the spiritual unity and solidarity of all existence.

ॐ ईशा वास्यमिदं सर्वं यत्किञ्च जगत्यां जगत् ।

(Om Isavasyamidam sarvam yatkinca jagatyam jagat;) 1

‘Whatever there is changeful in this ephemeral world, all that must be enveloped by the Lord.'

The whole universe, it tells us, is filled with the spirit of God.

Reference: The Message of the Upanishads by Swami Ranganathananda (p.1,75)