Katha-2-3-2-yadidaṃ kiṃ

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

Com.—If it be said that Brahman, the source of the world, by knowledge of which men are said to become immortal, does not exist and that all this has come out of nothing, it is not sound; all this universe, the highest Brahman existing, moves; and having come out of that alone, acts regularly. This Brahman, the cause of the origin, etc., of the universe is mahatbhaya, i. e., great and terrible. It is like the thunderbolt uplifted; as, at the sight of the master with the thunderbolt raised in his hand, the servants regularly keep his commands, so this world, with its lords and with the sun, moon, planets, constellation and stars, etc., regularly and without a moment’s respite obeys the law. This is the drift. Those who know this, this highest Brahman, the witness of all the modifications of one’s mind, become immortal.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda