An antithesis is used when the writer employs two sentences of contrasting meanings in close proximity to one another. Whether they are words or phrases of the same sentence, an antithesis is used to create a stark contrast using two divergent elements that come together to create one uniform whole. An antithesis plays on the complementary property of opposites to create one vivid picture. The purpose of using an antithesis in literature is to create a balance between opposite qualities and lend a greater insight into the subject.
When Neil Armstrong walked on the moon it might have been one small step for a man but it was one giant leap for mankind.