A wonderful overview:
"The earliest statement of the Nature of Reality occurs in the first book of the Rig-Veda: Ekam Sat-Viprah Bahudha Vadanti. "The ONE BEING, the wise diversely speak of."
The tenth book of the Rig-Veda regards the highest conception of God both as the Impersonal and the Personal: The Nasadiya Sukta states that the Supreme Being is both the Unmanifest and the Manifest, Existence as well as Non-existence, the Supreme Indeterminable"....
continues at:
http://www.swami-krishnananda.org/disc/disc_14.html