Kapadia's investigation of the Zoroastrian faith is important for clarifying and defining the important religious tenets native to followers of Zoroaster. Zoroastrianism is most famously known as the world's first major monotheistic religion. Zoroaster's assertion, made in his home country of Persia sometime between 1500 and 650 BC, was that there was only One God who governed over everything in the known Earth and wider universe. Over centuries, Zoroaster's creed became dominant in what is now modern-day Iran. In the ancient era, the belief in a single, omnipresent God was radical and profoundly different from the paganism which had come before it. Many people - including those in Greece and Rome - continued to worship a pantheon consisting of many Gods, corresponding to the elements of the physical world, for many centuries after Zoroaster perished.