Taittirīyopaniṣat 02.13

Now, therefore, the following questions arise: Does an ignorant person, after departing from this world, go anywhere? Or does a knower, after departing from this world, truly attain that world?

Taittirīya Bhāshya 02.13

Testimonial continues, where Varuna asks Lord Brahma, "Among the various paths employed by different beings, does everyone attain the same goal?" Lord Brahma responds, saying not everyone attains the highest goal. There is gradation even among the knowers based on their limited knowledge. Sound 'U' symbolises this response'. Then he elaborated on the knowledge of Brahman, symbolised by the sound 'Om'.

After this was spoken by Lord Brahma earlier, Varuṇa questioned the all-pervading Brahma.

Does whoever, even if ignorant, somehow attain Brahman? Is it only the knower, or perhaps all, or even some? There, some people do so properly, others improperly; thus even the wise. If all, then there too, would all properly attain? When asked, Brahmā, the four-faced one, spoke:

The ignorant never attain Brahman, while all the wise do. Yet, even among them, true attainment is always and only that of Lord Brahmā.

Lord Brahmā, intending that the attainment of others is by gradation and limited to particular happiness, indeed summarized this by the syllable 'u'.

After giving the question and answer, he then explained everything in detail. He said that only someone who truly understands may attain that, as stated. Then, in the same way, Brahman spoke the syllable 'Om'.