“There is something that appeals to our imagination also in the realization that this god, who is older than Brahma, Vishnu, and Śiva, still has his worshipers. No other god, unless it be the rather impersonal Heaven of the Chinese, has been revered with uninterrupted devotion for so many centuries. The gods of Egypt and Babylon were born earlier perhaps, but they all died long ago. Indra, worshiped to-day, was already a notable god fourteen hundred years before the Christian era. His contemporaries, Varuṇa, Mitra, and the ‘healing’ Twins, who correspond to the Dioskouroi, have long since vanished from the mind of the people. But Indra perdures…”
And a few paragraphs later, this relevant observation is made:
“The expression ‘when it rains’ is indifferently ‘when the god rains’ or ‘when Vāsava (Indra) rains.'”
-E. Washburn Hopkins, 1916, from the Journal of the American Oriental Society.
The rest of Hopkins’ essay is available here.