Katha-1-2-7-śravaṇāyāpi

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

Of thousands who seek good, it is some one like you who becomes the knower of the âtman; for, even for hearing, the âtman is not attainable by many; many others, though they hear of him, do not know the âtman, not being entitled, because their minds are not purified; again, the expounder of the îtman is, like a wonder, some one among many, Similarly, even among many who have so heard, some one alone of many, that is able, attains the âtman; the knower of the âtman is a wonder—some me who is instructed by an able preceptor.

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