Hari Om!
Before exploring the mischievously complex beauty of this Upanishad, Swāmī Aparājitānanda jī gives a beautiful elaboration of UPANISHAD in its multiple facets, viz. many meanings, content and results if its study.
IN GENERAL:
• Each Veda contains 4 divisions meant for 4 Āshramas and has a different Shanti Mantra.
• Each Upanishad (major or minor) is equally important.
• Upanishads deal with Brahma-vidyā or Self Knowledge.
Reasons for Ādi Shankarāchārya jī's commentary:
SPECIFICALLY:
• Source of name of this Upanishad
• It contains 18 mantras which are not simple single liners, but in form of couplets or stanzas of wonderful poetry, complicated content and many interpretations.
• Though first in the list of major Upanishads, it is not advisable to study it first.
• It comes in the Mantra portion of Shukla Yajurveda which was revealed to Yajñavalkya Rishi by Sun God. It is meant for rare seekers who go directly from being Brahmachārī to Sannyāsī.
Swāmījī unfolds the unique feature and status of the SHĀNTI MANTRA of this Upanishad with utmost ease and beauty.
• This Shānti Mantra is a Mahāvākya
• It starts with a paradox of highest interest.
• Practical interpretation of this Shānti Mantra starts magically with Swāmījī's remark: "Accept it or not, believe it or not, know it or not".
• Tip or technique of meditation of Shānti Mantra is to redefine existence as feel or experience from point of view of Nidhidyāsana and to meditate on any one term from Sat Chit Ānanda; the other two naturally combine to give resultant Shānti.
• 'Shānti' beautifully defined at three levels and in four stages.
(Īshāvāsya Upanishad - Talk 1 of 7 - Introduction and Shānti Mantra)