गोविन्द भाष्य (श्रील बलदेव विद्याभूषण - अचिन्त्यभेदाभेद वेदान्त ब्रह्मसूत्र भाष्य सटीक)

Purport by Śrīla Baladeva Vidyābhūṣaṇa
Because it makes the entire universe tremble, this vajra must be the Supreme
Personality of Godhead. This is so also because of the following statement of
Brahma-vaivarta Purāṇa:
cakraṁ caṅkramaṇād eṣa
vajanād vajram ucyate
khaṇḍanāt khaḍga evaiṣa
heti-nāmā hariḥ svayam
"Because He goes (caṅkramaṇa) everywhere He is called "Cakra" (moving in a
circle). Because He moves about (vajana He is called "Vajra" (thunderbold).
Because He cuts apart (khaṇḍana) the demons He is called "Khaḍga" (sword).
These are names of Lord Hari."
Also, because the word prāṇa (breath) and the word bhaya (fear) are used, the
passage must refer to the Supreme Personality of Godhead. In these ways it is
established that the word vajra here refers to the Supreme Personality of Godhead.
Sūtra 40
jyotir-darśanāt
jyotiḥ-effulgence; darśanāt-because of seeing.
It is so because the vajra is described as jyotiḥ (splendor).
Purport by Śrīla Baladeva Vidyābhūṣaṇa
Before the passage discussed is the following statement (Kaṭha Upaniṣad
2.2.15): na tatra sūryo bhāti na candra-tārake (When He does not shine, then
neither sun, moon, nor stars show their splendor). After the passage discussed is
the statement (Kaṭha Upaniṣad 2.3.3) bhayād asyāgnis tapati (Out of fear of Him
fire glows). In both these passages the Supreme Personality of Godhead is
described as transcendental splendor, and therefore the passage describing the
vajra (thunderbolt) between these two passages, must refer to the efulgent
Supreme Personality of Godhead.