This is the 10642nd most frequent Swedish word.
kristendom
Christianity.
Here, 'kristendom' is used as the name of the religion itself, illustrating its general usage in a broad context.
Kristendom är en av de största religionerna i världen.
Christianity is one of the largest religions in the world.
In this sentence, 'kristendomen' is in its definite form, referring specifically to Christianity in a detailed context about its historical progression.
Jag studerade kristendomens utveckling genom historien.
I studied the development of Christianity throughout history.
Here, 'kristendomen' refers to Christianity in a possessive sense, indicating its connection to cultural practices.