“Reason” was the ultimate court of appeal for deists. Tindal presents a Lockean definition of reason, self-evident truth, and the light of nature:
By the rational faculties, then, we mean the natural ability a man has to apprehend, judge, and infer: The immediate objects of which faculties are not the things themselves, but the ideas the mind conceives of them…. Knowledge [is]… the perception of the agreement or disagreement of our ideas. And any two of these, when joined together so as to be affirmed or denied of each other, make what we call a proposition… Knowledge accrues either immediately on the bare intuition of these two ideas or terms so joined, and is therefore styled intuitive knowledge or self-evident truth, or by the intervention of some other idea or ideas …… this is calleddemonstrative knowledge…
If there were not some propositions which need not to be proved, it would be in vain for men to argue with one another [because there would be no basis for demonstrative reasoning] … Those propositions which need no proof, we call self-evident; because by comparing the ideas signified by the terms of such propositions, we immediately discern their agreement, or disagreement: This is, as I said before, what we call intuitive knowledge…. [Intuitive knowledge] may, I think, be called divine inspiration as being immediately from God, and not acquired by any human deduction or drawing of consequences: This, certainly, is that divine, that uniform light, which shines in the minds of all men…
—Matthew Tindal, Christianity as Old as the Creation (II)
Deists did appeal to “the light of nature” to support the self-evident nature of their positive religious claims.
By natural religion, I understand the belief of the existence of a God, and the sense and practice of those duties which result from the knowledge we, by our reason, have of him and his perfections; and of ourselves, and our own imperfections, and of the relationship we stand in to him, and to our fellow-creatures; so that the religion of nature takes in everything that is founded on the reason and nature of things. I suppose you will allow that it is evident by the light of nature that there is a God, or in other words, a being absolutely perfect, and infinitely happy in himself, who is the source of all other beings….
—Matthew Tindal , Christianity as Old as the Creation (II)
Once a proposition is asserted to be a self-evident truth, there is not much more to say about it. Consequently, deist authors attempted to use reason as a critical tool for exposing and rejecting what they saw as nonsense. Here are two typical examples. The first is from John Toland’s Christianity Not Mysterious.
I hope to make it appear that the use of reason is not so dangerous in religion as it is commonly represented. …
There is nothing that men make a greater noise about than the “mysteries of the Christian religion.” The divines gravely tell us “we must adore what we cannot comprehend.” Some of them say the “mysteries of the Gospel” are to be understood only in the sense of the “ancient fathers.” … [Some] contend [that] some mysteries may be, or at least seem to be, contrary to reason, and yet received by faith. [Others contend] that no mystery is contrary to reason, but that all are “above” it.
On the contrary, we hold that reason is the only foundation of all certitude, and that nothing revealed, whether as to its manner or existence, is more exempted from its disquisitions than the ordinary phenomena of nature. Wherefore, we likewise maintain, according to the title of this discourse, that there is nothing in the Gospel contrary to reason, nor above it; and that no Christian doctrine can be properly called a mystery. …
Now, as we are extremely subject to deception, we may without some infallible rule, often take a questionable proposition for an axiom, old wives’ fables for moral certitude, and human impostures for divine revelation….
I take it to be very intelligible from the precedent section that what is evidently repugnant to clear and distinct ideas, or to our common notions, is contrary to reason. … No Christian that I know of expressly says reason and the Gospel are contrary to one another. But very many affirm that … according to our conceptions of them [i.e. reason and the Gospel] they seem directly to clash. And that though we cannot reconcile them by reason of our corrupt and limited understandings, yet that from the authority of divine revelation we are bound to believe and acquiesce in them; or, as the fathers taught them to speak, to “adore what we cannot comprehend.” This famous and admirable doctrine is the undoubted source of all the absurdities that ever were seriously vented among Christians. Without the pretense of it, we should never hear of transubstantiation, and other ridiculous fables of the Church of Rome. Nor should we be ever bantered with the Lutheran impanation….
The first thing I shall insist upon is that if any doctrine of the New Testament be contrary to reason, we have no manner of idea of it. To say, for instance, that a ball is white and black at once is to say just nothing, for these colors are so incompatible in the same subject as to exclude all possibility of a real positive idea or conception. So to say as the papists that children dying before baptism are damned without pain signifies nothing at all.
—John Toland, Christianity Not Mysterious:or, a Treatise Shewing That There Is Nothing in the Gospel Contrary to Reason,Nor above It (1696)
I have known some, who have alleged as a reason why they have forsaken the Christian faith, the impossibility of believing. Many doctrines (say these) are made necessary to salvation, which ’tis impossible to believe, because they are in their nature absurdities. I replied, that these things were mysteries, and so above our understanding. But he asked me to what end could an unintelligible doctrine be revealed? not to instruct, but to puzzle and amuse. What can be the effect of an unintelligible mystery upon our minds, but only an amusement? That which is only above reason must be above a rational belief, and must I be saved by an irrational belief? … You all agree that the belief of your Trinity is absolutely necessary to salvation, and yet widely differ in what we must believe concerning it; whether three Minds or Modes, or Properties, or internal Relations, or Oeconomies, or Manifestations, or external Denominations; or else no more than a Holy Three, or Three Somewhats… If I should be persuaded that an explanation of the Trinity were necessary to save my soul, and see the Learned so widely differing and hotly disputing what it is I must believe concerning it, I should certainly run mad through despair of finding out the Truth…
—William Stephens, An Account of the Growth of Deism in England (1696), pp. 19-20′