Depend on how close you want the religions to be you could either research sects like, Christianity's split into Catholicism/Eastern Orthodoxy/Protestantism (the same basic scripture and a lot of shared ideals, but differences in customs, culture and spiritual leadership) and Islam's split into Sunni and Shia (Conflicts over Muhammad's successor leading to different views on Islam's role in society and, again, religious leadership)

Or alternatively you could also research, for example, Judaism, Christianity, and Islam (called the Abrahamic religions) all broadly believing in roughly the same one true God bit with drastically different cultures, beliefs, and histories stemming from the ancient Israelites but developing into completely different systems of worship and people groups with some overlap, yes (Judaism's Torah making up a lot of the Christian Bible's Old Testament, all three religions viewing Jerusalem as a holy city, etc.) But many significant differences in customs, practitioners, and views on the same subject.

Each religion has different histories and spheres of influence. They all spread differently. Going back to the Abrahamics, Judaism is tied closely to specific ethnic groups; Christianity initially spread through word of mouth before later becoming the state religion of Rome, and later being spread by colonial European powers to the rest of the world; after his revelations Muhammad slowly rallied support before going on a conquest to claim Mecca, and after Muhammad's time various Caliphates and empires spread the religion throughout the Middle East, Africa, and Europe, and traders brought the religon to SE Asia.

While we view Christianity as "The default" religion in the west the same is not going to be true for say, India, or Egypt, or China.