Complex sentence: Encyclopedia Britannica (Eighteenth Edition) asserts that Mithraism was put together by an unknown "religious genius" in the Roman Empire during the 1st Century, presumably to furnish the young Empire with a compliant cult to inspire its soldiery.
Split into simpler sentences: Encyclopedia Britannica (Eighteenth Edition) asserts that Mithraism was put together by an unknown "religious genius" in the Roman Empire during the 1st Cent. CE, presumably to furnish the young Empire with a compliant cult to inspire its soldiery.