This is from p. 333 of Minority Report: H. L. Mencken’s Notebooks:
Religion, of course, does make some men better, and perhaps even many men. There can be no doubt of it. But making them better by filling their poor heads with grotesque nonsense is an irrational and wasteful process, and the harm it does greatly outweighs the good. If men could be made better—or even only happier—by teaching them that two and two make five there would be plenty of fools to advocate that method, but it would remain anti-social nonetheless. If the theologians could only agree on their doctrines their unanimity might have some evidential value, just as the agreement of all politicians that the first duty of the citizen is to obey them and admire them has some evidential value. It may not be true, but it is at least undisputed by all save a small fraction of heretics, which is certainly something. Fortunately for common sense, the theologians are never able to agree. Even within the sects, and under the more rigid discipline, there is constant wrangling, as, for example, between the Jesuits and the Dominicans. Thus the cocksureness of one outfit is cancelled out by the ribald denial of all the rest, and rational men are able to consign the whole gang to statistics and the Devil.
(h/t Jerry Coyne who is running a series this week on Mencken, calling him the first New Atheist.)