Provide a direct quotation if you want to provide evidence that someone made a certain claim—for instance if your intended readers might doubt whether the person actually made the claim in question.  Provide a direct quotation if the exact wording of a phrase, sentence, or passage is especially important—for instance if you are going on to discuss subtleties of its interpretation.  Provide a direct quotation if it is brief and you are going to be discussing that passage for the rest of your paper.

Otherwise, put the thought in your own words. This way, you demonstrate the depth of your understanding of the passage. (Incidentally, you'll find that sometimes you are able to craft a version that is clearer than the original, or a version that emphasizes aspects that are important for your paper.)