Bhagavad Gita · Jnana Karma Sannyasa YogaChapter 4, Shloka 2 (4.2)

How is knowledge lost?

• Shankaracharya: vicchinna-sampradāyaḥ saṃvṛttaḥ — the lineage was broken; the reason: durbalān ajitendriyān prāpya — it came to weak people who had not conquered their senses.
• Acharya Ramanuja (combined commentary on 4.1–4.2): tat-tac-chrotṛ-buddhi-māndyāt vinaṣṭa-prāyaḥ abhūt — it was almost destroyed because of the dullness of understanding of its various listeners. (In the corpus, Ramanuja's commentary on 4.2 simply repeats that on 4.1.)

"Parantapa": In this very verse Shankaracharya gives the derivation: ātmanaḥ vipakṣa-bhūtāḥ parā ity ucyante, tān śaurya-tejo-gabhastibhiḥ bhānur iva tāpayati — enemies are called "para"; he is the one who scorches them, like the sun, with the rays of his valour.