Mundaka-3-2-9-sa yo ha vai tat

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

Com.—It may he said that numerous obstacles are well known to exist in the attainment of good and that even the knower of Brahman may, therefore, be impeded either by some grief or other, or be made to take some other course by some other being, such as the Devas, reach some other after death and not reach Brahman. This cannot be; for all obstacles have already been removed by knowledge. Emancipation knows only the obstacle of ignorance and no other obstacle; because it is eternal and is being the Atman itself. Therefore, he in the world who knows that highest Brahman, as “I am directly that” does not take any other course. It is impossible even for the Devas to throw any obstacle in his attempt to reach the Brahman, because he becomes the Atman of all these; therefore he who knows the Brahman becomes even Brahman. Moreover in the line of this knower, there will not be born any who knows not the Brahman; again, he overcomes even during life the heart-burning caused by frustration of his many desires, crosses over karma known as vice and virtue, and being freed from “the knots of the heart” caused by ignorance, becomes immortal. It has already been said “the knot of the heart is untied, etc.”

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