Studying the history of theology involves tracing the origins of the world's major religions, as well as examining their sacred texts, and how their belief systems have changed and diversified over the ages. This should include Judaism; though one of the smallest religions, it is also considered the world's oldest organized religion stretching back to 2,085 BC. Beliefs centered around the top four religions by number of adherents should also be studied including Christianity, Islam, Buddhism, and Hinduism, whose followers combined make up 80% of the total world population as of 2011.
Learning about the history of theology can be a very complex and research-intensive practice due to the fact that human views on the nature of God and mankind's relation to the universe can be so disparate. There are estimated to be about 20 major religious groups in the world as of 2011 that have at least one million members each, but this number diverges wildly when individual sects are examined and smaller religions are included. Among Christianity alone, there are estimated to be over 34,000 groups with distinctly different beliefs and values.
The ancient nature of most major religions also requires that a theology education incorporate a good deal of general study in history for the time periods and cultures from which religions arose. Hinduism is traced back to 1,500 BC on the Indian subcontinent, and Islam has come to dominance in the Middle and Near East as well as large portions of North Africa since its founding by Mohammed in 610 AD. Buddhism and Taoism can be traced back to closely-related origins in history, with Gautama Buddha creating the foundation for Buddhist beliefs in 560 BC and Lao Tzu's teachings of Taoism occurring around 550 BC. Both such religions have widespread prevalence throughout China, Japan, and other Asian nations, and their diverse cultures have influenced them over the past 2,500 years.
Several theological movements have had periods where major offshoots of the primary religious group occurred. These tend to be defining moments in history that must be understood in the proper context when learning about the history of theology as well. Examples of this include Martin Luther's initiation of the Protestant Reformation in 1517 AD that led to the formation of hundreds of sects of Christianity, and the Sikh rebellion against Hindu polytheism in the 15th century.
Many religions also have their origins in ancient folklore and mythology, and these elements of human society have a strong and sometimes hidden or subtle influence on contemporary religious beliefs. A good example of a widespread religion traced back to ancient folklore is that of the Shinto belief system centered in Japan. Shintoism spans a spectrum of moral values for behavior governing everything including romance and the honoring of veterans through war memorials, to ritualistic agricultural practices. Theology courses that include gaining an understanding of Shinto beliefs in the history of theology must, therefore, account for transformational changes in Japanese culture, such as when it went from being an isolated nation to having relations with the west in the mid-19th century.
The history of theology is in constant motion, and often profoundly affected and defined by peripheral changes in human society. Advances in science and technology, medicine, and the discovery of new lands and mingling of cultures have affected the history of theology throughout time. As humanity has gained astronomical knowledge, or as the western and eastern cultures of Greece and Persia met, or the Portuguese colonized Mexico, the history of theology has been transformed and rewritten.
