After seeing this answer, I wish to know whether there are any verses declare Agni to be Highest Brahman (since he is frequently mentioned as Indra). Are there any such verses?
There are Plenty of verses. I'm just selecting a few of them from RigVeda.
“त्वमघ्न इन्द्रो वृषभः सतामसि त्वं विष्णुरुरुघायो नमस्यः
त्वं ब्रह्मा रयिविद् ब्रह्मणस् पते त्वं विधर्तःसचसे पुरन्ध्या " (RigVeda 2.1.3)
“Hero of Heroes, Agni! Thou art Indra, thou art Viṣṇu of the Mighty Stride, adorable: Thou, Brahmaṇaspati, the Brahman finding wealth: thou, O Sustainer, with thy wisdom tendest us”.
“तव प्रशास्त्रं त्वमध्वरीयसि ब्रह्मा चासि घृहपतिश्च नो दमे ” (RigVeda 2.1.2)
“Thou art Director, thou the ministering Priest: thou art the Brahman, Lord and Master in our home”.
“राजन्तमध्वराणां घोपां ऋतस्य दीदिविम् वर्धमानंस्वे दमे ” (RigVeda 1.1.8)
Ruler of sacrifices, guard of Law eternal, radiant One, Increasing in thine own abode”.
From the above it is clear that Agni himself is gods like Indra, Vishnu, Brahmanaspati, etc.... He is the director and ruler of sacrifices. He is also the guard of eternal law. Not only this Agni is all Gods also. Shatapatha Brahmana of YajurVeda 220.127.116.11 states:
- Having brought water forward, he takes out (the material for) a cake on eleven potsherds for Agni and Vishnu; for Agni is all the deities, since it is in Agni that offering is made to all the deities. Moreover Agni is the lower half, and Vishnu is the upper half of the sacrifice: 'I will become consecrated after encompassing all the deities, after encompassing the entire sacrifice,' thus he thinks, and hence there is a cake on eleven potsherds for Agni and Vishnu.
The Sanskrit word used is 'agnir vai sarvA devatA' ie. 'Agni is all Gods.'