Brahman is the ultimate reality; it is simultaneously Saguna and Nirguna; divisions are due to ignorance. Mind and intellect can never catch hold of it; they have only one option and that is to merge with it.
The gods are fond of the cryptic and dislike the evident.
Upaniṣhads are the first, foremost, and final words, given to the Universe from the Universe, given to humanity from the Cosmos. Each Upanishad mantra is a Cosmic Cognition for you to live as Cosmos.