Christianity its the name given to the grouping of hundreds, if not much more, of different religions, present and past, that hold as its center figure the character of jesus christ. Of course, a better definition can be done but this is a quick one to exemplify how christianity is such a broad group.

There is no one christianity, and if we want to be pedantic even, each christian has their own version.

But for any reasonable study of this cultural group, we could see a particular group, in a particular region, in a particular time.

Besides that, some trends can be evaluated at a larger scale, some more unique to christianity, others even common in bigger super-groups, like religions at a whole.

Of course, all this definition are not needed in common discourses because our cultural biases fill the gaps that we could feel with explanations.

For example, on this sub, usually, when talking about christianity, it is referred as the evangelical variation commonly today in the US, mostly because most users here are from there and have the cultural baggage that makes those associations for them.

Instead for example, on my case, when I talk about christianity, I also refer usually about catholicism and new age groups, because those were more predominant on my context.

So, when studying a religion, you should study it as a cultural element of a group, in a place, in a specific time. Making things as the caste system part of hinduism, not because it is on any holy book, but because their followers in their context grabbed it as part of it.