It was in England that the religion of reason was first consistently worked out… By the end of the 17th century most intelligent religious leaders were divided into two camps. Both agreed that the core of religion was a set of doctrines that could be established by the unaided natural reason. Both orthodox and radicals accepted as fundamental the religion of nature or reason.

The orthodox insisted also upon the importance of revelation. They were supernatural rationalists… The radicals, who were known as Deists, differed from them in rejecting revelation entirely, and insisting on the sufficiency of natural and rational religion…
Minds, in France or England, captivated by the new scientific method built up a religion of reason…

[These] convinced Newtonians, believed in the methods of scientific rationalism and in the world-machine that was their outcome. All of them agreed that religion is not an instinctive need and activity of the human soul, but essentially a science like physics, that is, a system of rational propositions given from without and to be tested as any other propositions are tested, by the evidence of the human reason…

These propositions are three: there is an omnipotent God, he demands virtuous living on the part of man in obedience to his will, and there is a future life in which he will reward the virtuous and punish the wicked. Man, employing his faculty of drawing conclusions from given premises, will thus see the advantages of living a righteous life, and will rationally order his life to attain a reward in heaven.

This simple creed remained throughout the century as the content of rational religion… It contained, nothing else. This creed was accepted, by orthodox and radicals together, as the essential content of the religious tradition of Christianity…

[The religion of reason became a] philosophical system appealing to the cool and deliberate reason of the man of common sense… For one who accepts Newtonian physics… it is impossible to prove that any given event was supernaturally produced. Whatever its cause, it is far easier to believe it effected by some natural factor…

The supernatural rationalists were refuted, and Deism, the pure religion of nature, was alone left with an argument to stand upon. That it too soon crumbled, and that religion has since been forced to rely, not upon any rational proof, but upon some kind of faith or mystic intuition, was due to the fact that before the Deists had well concluded their attack upon revelation, a more thoroughgoing rationalism had launched its arrows against natural religion itself.

This second stage of the rationalistic religious debate in the eighteenth century was thus no longer over the question of whether a reasonable man should believe in revelation in addition to natural religion: it was whether such a man should or should not believe in natural religion itself. For the first time serious attention was forced upon the arguments in support of the cardinal tenets of natural religion.

To understand the attacks made on natural religion by these complete skeptics, it is necessary to examine first the reasons given, by both supernatural rationalists and by Deists, for believing in its tenets. To the mind familiar with the modern conception of the world, that contains as a fundamental factor the notion of evolution and development, it is difficult to feel the force of these eighteenth-century arguments. To realize their cogency, it must be remembered that the whole age had no conception of the universe as a growing organism, but thought of it rather, following Newton, as a machine, in which time could cause no changes of structure. In such a Newtonian science the arguments of the advocates of natural religion were not only not absurd, but were genuine scientific hypotheses. They were not the arguments that would have appealed to preceding ages, nor do they appeal to the nineteenth century with its changed science; they were possible only in the Newtonian world, and there they seemed almost forced on men’s minds.
These arguments can be reduced to two: that from the necessity of a first cause, and that from design…

We can find both well stated, as implied in mathematical physics, by Newton himself. Such a harmonious and orderly machine as he had discovered the world to be, taken in conjunction with the fact that it seemed always, from its first beginnings, to have existed in its present form, appeared to him to demand an intelligent Creator to construct it. The very conception of the world as a machine or a complex watch implies a machinist or watchmaker to build it and to plan its intricate harmony and order. Watches and machines, in our experience, do not just happen; they are made, and they are intelligently made, to fulfill a definite purpose… The function of God became for them [the Deists] simply that of starting the machine in the first place; since then, God has not needed to concern himself with the operation of his perfect creation, and his sole value intellectually, aside from giving a scientific explanation of the origin of things, was to guarantee that the world was operated upon a moral basis, that it was permeated by a moral order that would punish in hell the unrighteous and reward the righteous…

It was inevitable that since God came more and more to be identified with the mathematical order of nature, he should lose any moral quality whatsoever, once the consequences of this were consistently worked out. Spinoza, a century before, had done so, and arrived at precisely this conclusion, that nature has nothing to do with human standards of right and wrong; and it was probably just because the Deists realized that their logic would lead them here that they so hated and shunned Spinoza… But for the most part the Deists closed their eyes to such disagreeable logic, and tried their utmost to worship harmony and order as supremely good.

Such were the arguments of the upholders of natural religion in favor of their creed. Obviously when the attack had once shifted to a questioning of this reasoning, it was not difficult to sweep it away by the same methods that the Deists had employed against revelation. This was done by two groups: the convinced skeptics and materialists, and the traditionalists who thought that by showing the inconsistencies of natural religion they could convince men that it was as shaky as revelation. This these latter did; but they did not find men adopting their corollary, that therefore both revelation and natural religion must be accepted on faith…

The three great and conclusive summaries of all that could be said against natural religion, books which made it quite impossible for an intelligent mind any longer to attempt the apology for even rational religion by the customary arguments of the century, were written by Hume, by the Frenchman Holbach, and by the German Kant. Hume, in religious matters at least, was a typical skeptic: he refused to draw any positive conclusions from his destructive critique. Holbach was a convinced materialist and a good deal of a pantheist; while Kant, summing up the rationalistic attack on rational theology, also laid the foundations for the various attempts of the nineteenth century to establish religion upon feeling and intuition and some special religious sense — attempts which, however successful in themselves, at least have avoided the keen edge of the rationalistic sword.

[Hume stated] We have no reason for concluding from a life in which rewards and punishments do not accord with human deserts, that there is another in which they do. Hume went on, in his Dialogues, to show that there could not even be any argument for the existence of an all-wise and all-good Creator. There is no necessity of the universe having had a first cause. It is as easy to conceive of it as self-existent and eternal as to assume an external cause with those qualities. There is no analogy between an object in the world, like a watch, and the entire world; we have seen watches made, but not worlds. Order may be as natural as chaos, and hence harmony and universal law need no further reason for their existence… If the universe did indeed have an author, he may have been an incompetent workman, or he may have long since died after completing his work, or he may have been a male and a female god, or a great number of gods. He may have been entirely good, or entirely evil, or both, or neither —probably the last.

Hume suggested doubts: he questioned the tenets of natural religion. Holbach categorically denied God, freedom, and immortality.
“The worshippers of a God find especially in the order in the universe an invincible proof of the existence of an intelligent and wise being who governs it. But this order is only a sequence of necessary motions produced by causes and circumstances which are now favorable and now harmful to us: we approve the first and complain of the second... To be surprised at seeing a certain order reigning in the world is to be surprised that the same causes produce constantly the same effects... What is order for one being is disorder for another.”

In a word, order and purpose is a man-made distinction that has no meaning in the world apart from man. Finally, as to the moral governance of the world and the righteousness of the power at work in the universe, Holbach says:
“More than two thousand years ago the wise Epicurus said: ‘Either God wants to prevent evil, and cannot do it; or he can do it and does not want to; or he neither wishes to nor can do it, or he wishes to and can do it. If he has the desire without the power, he is impotent; if he can and has not the desire, he has a malice which we cannot attribute to him; if he has neither the power nor the desire, he is both impotent and evil, and consequently is not God; if he has the desire and the power, whence then comes evil, or why does he not prevent it?’ For more than two thousand years the best minds have been waiting for a rational solution of these difficulties, and our doctors teach us that they will be removed only in a future life.”

It is perhaps worth remarking that Holbach combined with his atheism and materialism a singularly noble moral ideal of benevolence, justice, and humanity. To such men the discarding of traditional religion meant a liberation from superstition and the possibility of a genuinely enlightened and universal morality.

With Holbach, who represented in a frank way what most intelligent Frenchmen had by 1770 come to believe, we have arrived at a complete and thoroughgoing atheism and materialism. The course of our discussion has made it clear that the Age of Reason, starting with the religious assumptions natural in Newtonian science, was bound to develop just such a complete denial of every one of the tenets of traditional Christianity.

A hundred years earlier Spinoza, saturated with Cartesian science, had laid down a similar and on the whole more profound system; but men had not in his day really assimilated the principles of the new factor they saved the day for religious belief, and made possible the religious revival of the first half of the nineteenth century. The new source of religious truth acclaimed by Kant lies beyond our immediate subject here… Here we have only to note that Kant seemed to have disproved forever the possibility of a purely rational religion.

His demolition of rational theology was contained in his Critique of Pure Reason, published in 1781.
"I assert then that all the attempts at a mere speculative use of the reason in the field of theology are entirely fruitless and in their very nature null and void."
…While Deism and natural religion lingered on in some minds… to the vast majority of intelligent minds interested in religion it seemed that the primary task, in view of the abandonment of a rational basis of the religious life, was to effect a reconstruction on some non-rational or supernatural principle…

Thus the working-out of the principles of Nature and Reason, the cardinal ideals of the age that worshiped the Newtonian world-machine, when applied to the great Christian tradition, seemed wholly destructive, and the attempt to build a new scientific religion upon them completely failed. Multitudes, of course, were quite untouched by these lines of thought, just as they were quite impervious to the new scientific knowledge; but the thinking middle-class, to whom the future belonged, accepted them unreservedly, on the whole.

When the reconstruction of the Christian tradition and its adaptation to the new intellectual world was undertaken, it was with the clear understanding that the eighteenth century had made the foundation of religion upon the principles of scientific reason henceforth impossible.