Although theme is sometimes used in the same sense as motif to signify recurring concepts in literature, the term mainly refers to the argument or general idea expressed by a literary work, whether implied or explicitly stated.
Milton explicitly declares that the theme of Paradise Lost is to "assert Eternal Providence, / And justify the ways of God to men." But even where a single theme is stated, others can be seen as well--perhaps variations on a central idea. For instance, some other themes in Paradise Lost are those of pride and fall, the psychology of temptation, and the limited nature of human freedom. Generally, it is more accurate to refer to "a theme" than to "the theme," especially in the majority of works, in which no theme is stated.