Muhammad changed the Arabic religion from a largely polytheistic institution to a primarily monotheistic one. The religion Muhammad started, Islam, preaches the existence of only one god, Allah, while pre-Islamic Arabia had a multiplicity of deities.
Before Muhammad, the Arabian Peninsula consisted of a number of relatively independent cities along the coasts and rivers. Nomadic tribes lived in the desert in the center of the peninsula. This geographical and political division also led to religious division, with each city having its own pantheon of gods and goddesses in addition to greater gods and goddesses who influenced larger areas. In Mecca, for example, was a building called the Kaaba that housed the images of 360 local and regional gods. In this polytheistic mix, there were some Christian and Jewish populations as well. In Islam, this period of time is called "al-jahiliyya," or "the age or condition of ignorance."
Like Christianity and Judaism, Muhammad's religion, Islam, was monotheistic. Initially, the local authorities in Mecca did not accept the new religion, fearing that it would offend the local gods and create a decline in business and pilgrimage traffic. However, the religion began to spread rapidly, both by convincing new believers and by conquest. By the end of Muhammad's life, the entirety of the Arabian Peninsula was under Muslim control, and the people living in that area were largely monotheistic Muslims.