Yes, You are looking for Rigveda 1.32.1
The verse of Agnipurana you're talking about in the question might be Agnipurana 259.14 (259th chapter from Agnipurna is "Rigvidhanam") quoting from sa.wikisource.org: (Transliterated into IAST):
ṛgbhiḥ ṣoḍaśabhiḥ kuryyādindrasyeti dine dine |
hihaṇyastūpamityetajjapan śatrūn prabādhate || 259.14 ||
Yes, there is a Rig-Veda mantra that points towards Indra being alone at the beginning i.e. this mantra praises Indra as Supreme Creator and he was alone before. All the material forms /beings were created by him and all the visible universe came after Indra.
This Rig Veda mantra is from Mandala 8- Sukta 96- Mantra 6. The Rishi of this sukta is Dyutana, ...
The question is
So who was the earliest Vedic scholar according to secular history to
advocate the idea that Indra is the supreme Brahman?
We have to split the question into parts for understanding the issue in a better way.
The question was on advocating Indra as the Supreme Brahman
Earliest Vedic scholar
As far as ...
BhagavAn veda vyAsa considers Indra as the supreme brahman in the Mahabharata. Below is a prayer to Indra by Kadru -
Adi parva 21 (Mahabharata BORI edition)
tadavasthAnsutAndRRiShTvA kadrUH shakramathAstuvat || 6|| namaste devadevesha namaste balasUdana | namuchighna namaste.astu sahasrAkSha shachIpate || 7|| sarpANAM sUryataptAnAM vAriNA tvaM plavo ...
Is there any refutation of the idea that Indra is Brahman?
Yes, the Vedas do, and the Brahma sutras make this matter more clear.
The Upanishad passage in question is here:
Indra said: 'I am prâna, meditate on me as the conscious self (pragñâtman), as life, as immortality. Life is prâna, prâna is life. Immortality is prâna, prâna is immortality. As long ...
Because the Purāṇas came later, the question looks better if rephrased as: Why is Purāṇic Indra so different from Vedic Indra?
The late P. L. Bhargava addressed this very issue in his paper The Origin and Development of Purāṇas and Their Relation With Vedic Literature. As he says, the authors of various Purāṇas are responsible for bringing disrepute to ...
The story of Indra and Vṛtra is metaphorical as explained below.
Since you cited a quote from the Rig Veda. Therefore, in the first place, the question of Viṣṇu helping Indra has to be understood in the context of the primary sources namely Vedas and Brahmanas.
In Śatapatha Brāhmaṇa, it is stated that
वृत्रो ह वा इदं सर्वं वृत्वा शिश्ये । यदिदमन्तरेण ...
From what I remember from seeing on Vishnu Puran teleserial, Virochana knows about the threat to his life by Indra. He is advised to pray to Lord Surya for a crown that grants immortality to anyone who wears it. He obtains it after penance and Indra becomes even more insecure of this. Perhaps either on a pilgrimage or hunting trip he visits a hermitage where ...