Since you don't say "Brahma" or "Zeus" or "Yahweh", you've opened a new question. This word "God" is middle English, evolved from "guth", an androgyneous name for a Gothic deity. It appears to have evolved from the Indo-Iranian word for "pour", although it seems to have been exclusively used in translating the Greek word "Theos" which probably meant "to run" (or "the flow"). BTW Plato may have coined the term, in his account of the trial of Socrates, as a sort of literary device, to infer Zeus, while meaning something perhaps more akin to "creator God". In that case, all Christian Gods have an invention of Plato (or Socrates, if he's a real person) to thank for inventing their God. For the Goths, their invention of "The Pouring" occurred probably around 321 AD when the Codex Argenteus (oldest Gothic New Testament) was written. It was at the fall of Rome, or the Goths might not have been involved in this part of the puzzle. The Goths may not have understood much about Yahweh during the era ca 300-700 AD when "God" was coined to mean the Gothic Christian deity. Prior to that era it was the name of their deity, at times androgynous, and perhaps in its animist inventions (the trees, the water, etc.) to substitute for natural phenomena for which they had no science. Shortly after the "Guth" or "Gudth" or "Gud" came into use in this manner, the Vulgate began to propagate, and the Romanized name for Zeus and/or Theos, "Deus", would have been used in parallel among the Christianized Goths. So again "The Pouring" and the "The running" may have fused at that time. You use the word in a modern vernacular that is probably invented in the King James Anglicization of Theos (The Pouring) and/or Yahweh (a possible reinvention of the Phoenician deity, and Canaanite husband of goddess Ashereh). Under that criteria, God as you use the word was probably invented in ca. 1611 in England. It would be sometime later when God would be used generically, to span inventions as widely as from Yahweh to Brahma, for example. That would be a good exercise to figure out. Was it maybe in the era that England was losing its hold over India that "God" began to be used interchangeably with their deities?