The first Mandala ("book") of the Rigveda has 191 hymns. Together with Mandala 10, it forms the latest part of the Rigveda, its composition likely dating to the Early Iron Age.
Hymn 1.1 is addressed to Agni, arranged so that the name of this god is the first word of the Rigveda. The remaining hymns are mainly addressed to Agni and Indra. Hymns 1.154 to 1.156 are addressed to (the later Hindu god) Vishnu.índram mitráṃ váruṇam agním āhur / átho divyáḥ sá suparṇó garútmān ékaṃ sád víprā bahudhâ vadanty / agníṃ yamám mātaríśvānam āhuḥ "They call him Indra, Mitra, Varuna, Agni / and he is heavenly nobly-winged Garutman." "To what is One, sages give many a title / they call it Agni, Yama, Matarisvan." (trans. Griffith) – Rigveda 1.164.46
Hymns such as the above in Mandala 1 led scholars such as Max Muller to describe the theology of Vedic religion as a form of henotheism. Muller noted that the hymns of the Rigveda, the oldest scripture of Hinduism, mentions many deities, but praises them successively as the "one ultimate, supreme God", alternatively as "one supreme Goddess", thereby asserting that the essence of the deities was unitary (ekam), and the deities were nothing but pluralistic manifestations of the same concept of the divine (God).
The Vedic era conceptualization of the divine or the One, states Jeaneane Fowler, is more abstract than a monotheistic God, it the Reality behind and of the phenomenal universe, which it treats as "limitless, indescribable, absolute principle", thus the Vedic divine is something of a panentheism. In late Vedic era, with the start of Upanishadic age (~800-600 BCE), from the henotheistic, panentheistic concepts emerge the concepts which scholars variously call nondualism or monism, as well as forms of non-theism.