Religious scriptures
Before researching any religious references to the Firmament, I made an important note to myself to take on this journey with an open mind, knowing that I do not have access to any material first hand, but must use interpretations of the written text. Firstly I looked at verses from the Bible (Christianity), Koran (Muslim) and the book of Job (Hebrew).
I used Google as the fastest tool to obtain any references to the Firmament across all three religions and put my findings on a chart to understand the whole picture better, concluding that all three made references to it in one form or another.
Of course, there are different words that have been used to explain the Firmament, which are "Vault, Heaven, Ceiling, Metal Mirror and a Circle", all of which could point towards the Firmament existing. The connection between the Firmament and Flat Earth is uncanny, but seem to go hand in hand with each other. Having now established that there are historical and biblical references to there being a Dome above the Earth, it's now time to move onto the reasons why this belief system changed, and whom continued to purport the Firmament belief in a spherical world?