The Age of Terra, also sometimes called the Age of Progress in older Imperial texts, is the era of human history between the 1st and 15th Millennia (1 - 14,999 AD) when Mankind advanced from a pre-industrial Iron Age culture to become a star-spanning Fusion Age civilisation. The original reason for numbering the years from this point is because it was believed to be the year of the birth of the ancient religious leader Jesus Christ, which has long been forgotten by humanity, which now possesses only one religion -- a deep faith in the God-Emperor of Mankind and his Imperial Cult. During the long years of the Age of Terra, humanity came to dominate the Earth culturally and technologically. Myriad human civilisations came and went during this long span of now mostly forgotten human history.