As a group, the term is lowercase. When referring to a text, it's capitalized, as in Acts of the Apostles or Apostle to the Gentiles.

Do you capitalize Twelve Apostles?

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.

How does the Bible define disciple?

1 : one who accepts and assists in spreading the doctrines of another: such as. a Christianity : one of the twelve in the inner circle of Christ's followers according to the Gospel accounts.