It's even more interesting when you think about Christianity not as a clear category, but as a cloud of practices, beliefs and institutions in a broader family of religious patterns.

Mircea Eliade asks how Christianity reinterpreted sacred history, myth, salvation. What does Christianity do with motifs older than itself, such as paradise, rebirth, sacrifice? In A History of Religious Ideas [0], he treats the emergence and development of Christianity, including Judaism, early Christianity, Gnosticism, late antiquity, medieval religious forms and also how it interacted with other traditions. I think it complements quite nicely the geographical spread of Christianity by also clarifying what kind of transformations of religious symbols make it recognisable as Christianity across such different contexts.

There's also "Darwin's Cathedral" [1] that analyses religion as group-organizing system, with a focus on Calvinism. Didn't go through it, but seems relevant. It was recommended by Robert Sapolsky in his Introduction to Human Behavioral Biology lecture series [2].

[0] A History of Religious Ideas - Mircea Eliade

[1] Darwin's Cathedral - David Sloan Wilson

[2] https://www.youtube.com/watch?v=NNnIGh9g6fA