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.
The Greek word apóstolos means "one who is sent out," giving it a proper connection with the word apostle. The word’s original English meaning of "messenger" referred to the followers of Jesus. The modern meaning can be more secular, with the word appropriately being applied to any leader breaking new ground or supporting an important new idea. As an apostle of the latest developments in good nutrition, you were outraged at the sight of french fries. Clearly, you had no choice but to make them disappear.