When referring to religions such as Christianity, Judaism, Hinduism, Islam, Buddhism, etc. you should always capitalize the word since religions are proper nouns.

Is Christian capitalized as an adjective?

Christian is a proper noun as well as an adjective. The reason why it is capitalised even when it is employed as an adjective is because it's based on the proper name Christ.

Is Christian a proper noun?

The noun Christian is a proper noun. It refers to a person who adheres to a specific religion. Like the names of the religions themselves, the names...