What is the meaning of [Laws catch flies, but lets hornets go free]

“He complained in no way of the evil reputation under which he lived, indeed, all over the world, and he assured me that he himself was of all living beings the most interested in the destruction of Superstition, and he avowed to me that he had been afraid, relatively as to his proper power, once only, and that was on the day when he had heard a preacher, more subtle than the rest of the human herd, cry in his pulpit; “My dear brethren, do not ever forget, when you hear the progress of lights praised, that the loveliest trick of the Devil is to persuade you that he does not exist!”