China offers the longest continuous written record of any religious culture on earth, and that fact shapes everything the atlas can say about it.
It begins with divination. The Shang oracle bones, recognised for what they were only in 1899 and excavated at Anyang thereafter, are the working records of a royal divination bureau: the question put, the cracks read, and often the outcome. They are religious documents that survived because they were administrative ones. From there the record runs more or less unbroken through dynastic histories, ritual manuals, the Daoist canon and a Buddhist canon that was being printed while Europe was still copying by hand.
The distortion runs the same way as the advantage. This is the record of a literate class attached to a state, and it reports what that class cared about. Popular religion appears in it most clearly when the state was suppressing it, so a great deal of what is known about village cults, spirit mediums and millenarian societies comes from documents written to justify their prohibition.
Two further complications sit on top. Buddhism arrived as a foreign import and was argued about in exactly those terms for centuries, so texts about it are frequently polemics. And the twentieth century brought deliberate campaigns against religious practice, so the modern ethnographic record has a hole in it, and reconstruction after 1980 is itself now a subject of study.