Complex sentence: Social scientific assessment of the extent of "atheism" in various populations is problematic because in most of the world outside of Europe and East Asia most populations are believers in either a monotheistic or polytheistic system.
Split into simpler sentences: Social scientific assessment of the extent of "atheism" in various populations is problematic. First, in most of the world outside of East Asia most populations are believers in either a monotheistic or polytheistic system.