The concept of Ishwara being one supreme being, Lord and God is used in Puranic and Modern Hinduism.
But was such a concept present in Vedas or authentic Upanishads?
This Wikipedia article says no
The word Īśvara does not appear in Rigveda. However, the verb īś- does appear in Rig veda, where the context suggests that the meaning of it is "capable of, able to". It is absent in Samaveda, is rare in Atharvaveda, but it appears in Samhitas of Yajurveda. The contextual meaning, however as the ancient Indian grammarian Pāṇini explains, is neither god nor supreme being.
Did the concept of Ishwara, the Single Supreme God exist in Vedas?
One can readily give example of Purusha Sukta, which is clearly Monotheistic, but again this Wikipedia article seems fo say that Purusha Sukta was a later addition.
Some scholars state that certain verses of Purusha Sukta are later interpolations to the Rigveda. One of the reasons given is that it is the only hymn in all the Vedas that mentions the four varnas by name - although the word "varṇa" itself is not mentioned in the hymn
This four varna-related verse is controversial and is believed by many scholars, such as Max Müller, to be a corruption and a medieval or modern era insertion into the text
Many 19th and early 20th century scholars questioned as to when parts or all of Purusha Sukta were composed, and whether some of these verses were present in the ancient version of Rigveda. They suggest it was interpolated in post-Vedic era and is a relatively modern origin of Purusha Sukta
There can be little doubt, for instance, that the 90th hymn of the 10th book (Purusha Sukta) is modern both in its character and in its diction. (...) It mentions the three seasons in the order of the Vasanta, spring; Grishma, summer; and Sarad, autumn; it contains the only passage in the Rigveda where the four castes are enumerated. The evidence of language for the modern date of this composition is equally strong. Grishma, for instance, the name for the hot season, does not occur in any other hymn of the Rigveda; and Vasanta also does not belong to the earliest vocabulary of the Vedic poets.
— Max Müller
1. Is the concept of one Ishwara present in Vedas?
2. How old is Purusha sukta?