The purpose of a caption is to

provide a visual to support text.
describe an image in a text.
organize and sort information.
divide a book into logical units.

describe an image in a text.

Explanation:

It's the tiny thing underneath a picture

in paragraph 6 of "sinners in the hands of an angry god" by jonathan edwards, he talks about how doing good things alone will not save men from going to hell because they're at the mercy of the will of god. "spiritual reform" provides men with a false sense of safety.