Bhagavata Purana, 1.2.11
vadanti tat tattva-vidas
tattvam yaj jnanam advayam
brahmeti paramatmeti
bhagavan iti sabdyate
Translation:
Learned transcendentalists who know the Absolute Truth call this nondual substance Brahman, Paramatma or Bhagavan.
Notes:
* tattva-vidah means those who have knowledge (vid; veda also comes from the root vid=knowledge) about the truth, tattva (Absolute Truth in this case) and a choice of words for this expression was "learned transcendentalists";
* jnanam comes from root jna, similar to English know, Greek gno (like in gnosis) co-gnitio in Latin, znanye in Russian etc. and means knowledge;
* a-dvayam means non-dual;
* in sanskrit verse, there is no specific mention of a "substance", but it is a good choice to have in translation anyways. Sub = beneath, under; stance from Proto-Indo-European root sta = to stand. Interesting connection with English: under-stand. So, substance is used here not like in chemistry, but in a theological sense and is handy to describe what "That" (Tattva, God) is - He is truly a supporting, underlying principle of everything there is, same idea reflected in a name Vishnu - all pervading, sustainer and maintainer of all.
I would say that, according to this verse, God is One, He is only perceived from different perspectives, one being seeing Him as Brahman. We must have different perspectives, since He is too complex to comprehend from just one standpoint. Same thing with scientific approach to nature, which is so complex that we must address that same, one nature from different disciplines, like physics, chemistry, biology etc. Our understanding of nature is not only the sum of all the interdisciplinary data, but is like a synergy - all that data taken together give us a new quality, an enhanced vision of nature that goes beyond the mere sum of elements. We hope to derive that same synergy from our attempts to describe God.
You can tell the sensibility or devotional mood or spiritual realization of a person by analyzing how well he understands these perspectives and which one he prefers. You can also tell about his intelligence or the intelligence of the whole religious system, by how well it understands and presents this problem of knowing God. So, the above verse in fact better describes our position in relation to God, rather than God Himself. Like if it is more about us, then about God. God does not make distinction between those three aspects of Himself.