From George H Smith:
A major flaw in the contingency argument lies in its artificial dichotomy between necessary and contingent existence. To say that something exists contingently makes sense only within the sphere of volitional action. So, for example, we might say that a building exists contingently, meaning that, if certain men had decided to act differently, the building would never have been constructed. With this exception, however, the idea of contingent existence has no application. Everything exists necessarily. 17 In using the distinction between necessary and contingent existence as part of his argument, the theist smuggles in a crucial premise. He assumes that there are, in effect, two kinds of existence: deficient and sufficient. He then argues that the universe is metaphysically deficient, that it does not exist necessarily, so we must infer the existence of a transcendent necessary being. Thus, in his original distinction between necessary and contingent existence, the theist assumes beforehand that natural existence requires an explanation. In using the necessary-contingent dichotomy in his argument, the theist is asking that a major point of controversy be conceded to him without argument. If the dichotomy is challenged, the contingency argument can go nowhere. If one rejects the notion of contingent existence (in the sense here described), there is no reason to posit a transcendent, necessary being. As Copleston puts it, “if one refuses even to sit down at the chess-board and make a move, one cannot, of course, be checkmated.”
Smith, George H.. Atheism: The Case Against God (The Skeptic’s Bookshelf) (pp. 251-252). Prometheus Books. Kindle Edition.