Katha-2-3-4-iha cedaśakadboddhuṃ

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

If he is able to know and knows even during life, this Brahman_} the cause of fear, before the falling of the body, then he becomes freed from the bond of Samsâra. If he is not able to know, then, i.e., from want of that knowledge, he becomes able to take, i.e., he takes a body in earth and other worlds, where those who are fit to be created, are created. Therefore, before the falling of the body, attempt should be made to realise the âtman; for the realisation of the âtman even here will be clearly horrible as that of a face reflected in a mirror; not in other worlds except the Brahmalôka; and that is hard to reach.

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