Kena Upanishad comes second among the ten Upanishads (literally meaning ‘sitting near God’). This passage explains the mysterious nature of God.

Passage

If thou thinkest “I know God well”, then thou certainly knowest but little of God’s nature. What thou knowest of God, and what is known about Him among Devas, ( Learned people ), I think that it is indeed worth thinking about.

I do not believe that I know Him well, nor that I do not know Him. I know Him. He among us who says he knows Him ( fully ) knows Him not; he who says he does not know Him fully, knows Him.

He is understood by those who think they do not understand Him; he does not know who says he understands Him. He is unknown to those who profess to know Him, and is known to those who do not profess to know Him.

Knowledge of God derived by constant meditation leads to immortality. By self exertion man obtains strength, by knowledge ( of God ) he obtains immortality.

Pt. Ganga Prasadji. Kena Upanishad. Sarvadeshik Arya Pratinidhi Sabha, Delhi, 1900.