Syncretic religions are those religions that have come into being by combining two or more different belief systems into one. Many modern religions have syncretic origins and so do many ancient religions. The beliefs of the ancient Greeks and the ancient Romans were syncretic in nature as these cultures tended to absorb or nationalize the beliefs of the local people in areas they conquered.
Modern African-American religions and belief systems are often syncretic combining the original tribal beliefs of the people abducted from Africa with the Christian faiths of those who purchased them when they were sold into slavery in the Americas. Some of these also blended with Native American traditions as well. Some examples of these include Umbanda, Santeria, Candomble, Voodoo, and Rastafarianism.
While many syncretic paths were born out of conquest, some evolved more peaceably. Thelema is a blend of what was perceived as the best parts of several magico-religious traditions and Wicca is also a blend of several traditions as are many other NeoPagan and New Age movements.
Modern religious movements like Unitarian Universalism and Universal Sufism are consciously and purposely syncretic in their effort to be accepting of all paths.