When referring to a text, it's capitalized, as in Acts of the Apostles or Apostle to the Gentiles. When referencing a specific individual and preceding a name, it's also capitalized, because it's a formal title: the Apostle Paul.

Does Apostle have to be capitalized?

Although an apostle might be a pioneering supporter of any new cause, the word is most often used to refer to the twelve original disciples of Jesus who went on to spread the gospel. When used as a title, it's capitalized.

Do you capitalize prophet Isaiah?

Chicago lowercases generic descriptive titles, so we advise writing “the prophet Isaiah” or “the apostle Paul” just as we would advise writing “the artist Frida Kahlo” or “German chancellor Angela Merkel” (but “Chancellor Merkel,” in which the title is part of the name; see CMOS 8.21).