John Stuart Mill, answered from the texts and cited to the page.
The supernatural arguments for theism are weak: the design argument is the only one with any genuine force, and it establishes at most a finite, limited deity, not the omnipotent creator of orthodox religion. Nature, regarded as the design of a good and wise being, is gross and immoral — Nature kills, tortures, and destroys with massive indifference, and any theology that takes Nature as moral teacher is monstrous.
The defensible attitude toward supernatural theism is hope, not belief. The genuine religion is the Religion of Humanity (taken from Comte but transformed): the cultivation of moral feeling toward humanity itself, the consecration of human life to human improvement, without supernatural sanction or transcendent reward. This is what religion has always partly been; what remains when the supernatural element is critically reduced is not nothing but the moral substance.12
Positive value the argument of course has none. Without dwelling further on these or on any other of the a priori arguments for Theism, we will no longer delay passing to the far more important argument of the appearances of Contrivance in Nature.Three Essays on Religion, pp. 167–168
The limitation of his power more probably results either from the qualities of the material — the substances and forces of which the universe is composed not admitting of any jarrangements by which his purposes could be more completely fulfilled; or else, the purposes might have been more fully attained, but the Creator did not know how to do it; creative skill, wonderful as it ia, was not sufficiently perfect to…Three Essays on Religion, pp. 186–188
The Three Essays caused considerable disturbance on publication in 1874 — Mill's earlier reputation as a thoroughgoing rationalist had not prepared his readers for the qualified theism of Theism (the late essay). Modern readers split: those who emphasize the qualified theism see Mill as inconsistent with his earlier work; those who emphasize the Religion of Humanity see the Three Essays as the natural completion of the naturalist-utilitarian project. The contemporary scholarly consensus (Skorupski, Riley, Raeder) treats the position as continuous and coherent.