Questions & Answers about Kirken er gammel.
Because kirken is the definite singular form: the church.
In Norwegian, the definite article is usually attached to the end of the noun instead of being a separate word before it.
- kirke = church
- kirken = the church
So the -en ending here works like English the.
For many nouns in Bokmål, -en is the definite singular ending used with masculine-style forms.
With kirke, this is especially interesting because the noun can often be treated as either feminine or masculine in Bokmål:
- en kirke – kirken
- ei kirke – kirka
Both are possible in Bokmål. Your sentence uses the common form kirken.