Like many of his English contemporaries, Locke was deeply interested in matters of faith and religion. Keenly aware of the theological controversies of the day, he developed and defended views of his own that proved influential on the Deists of the next generation. Although knowledge of God is vital for human life and practical conduct, on Locke's view, it cannot be grounded legitimately on the supposedly universal possession of an innate idea. [Essay I iv 8-9] Although he claimed to demonstrate the existence of God as the only reasonable explanation for the emergence of thought in an otherwise material world, Locke warned against an excessive reliance upon non-rational considerations in the defense of particular religious doctrines.
We form our complex ideas of spirits, Locke held, by adding the notions of a variety of cognitive powers (themselves acquired, as ideas of reflection, from careful observation of our own mental operations) to the abstract idea of substance in general. Since this is perfectly analogous to the way we form complex ideas of bodies from our ideas of sensation, the results are perfectly comparable: although we are familiar with both bodies and spirits in our ordinary experience, the real essences of thinking and moving substances alike remain forever unknowable. [Essay II xxiii 22-25]
It is easy enough to employ an empiricist version of Descarte's cogito ergo sum as a demonstration of my own existence, of course. Thinking of any sort, feeling pleasure and pain, even the act of doubting itself, are all experiences that carry with them a full assurance of my own existence as a thinking thing. [Essay IV ix 3] Notice, however, that Locke declined the further implications of the Cartesian inference to sum res cogitans. Thinking, he argued, is merely an activity of the soul, not its essence; so the continued existence of an individual thinking thing does not necessarily entail its continuous consciousness. [Essay II i 10-16] Locke frequently expressed significant reservations about the demonstrabilityif not the very truthof Cartesian dualism. Given our widespread ignorance of the inner constitution and operation of substances generally, Locke notoriously suggested that, for all we know, the power of thinking could be providentially superadded to an organic human body as easily as separate thinking and material substances could be combined. [Essay IV iii 6] But the ultimate origin of thinking itself is another matter.
According to Locke, the existence of God is an instance of demonstrable knowledge in any reasoning being. Since I know intuitively that I exist as a thinking thing, and since nothing can be made to exist except by something else which both exists and has powers at least equal to those of each of its creations, it follows that from all eternity there must have existed an all-powerful cogitative being. [Essay IV x 3-6] This amounts to a variation on the Aristotelean / Thomistic cosmological argument for God's existence: there is an instance of thinking; every thinking thing proceeds from some other thinking thing; but there cannot be an infinite regress; so there must have been a first thinking thing, or God.
What most interested Locke in this argument was its emphasis on thinking. Willing though he was to contemplate the possibility that individual human beings are animal bodies that have the power to think, Locke insisted that mere matterwith its primary qualities of solidity, mass, and motioncould never on its own give rise to cogitative activity of any kind. Although it may not strictly be inert, matter is most definitely unthinking. [Essay IV x 9-10] Materialism can never account for the emergence of thought in a universe containing only senseless matter. Thus, from the fact that there is now thinking in the universe, it follows that there always has been thinking in the universe; the first eternal being from which all else flows must itself be a thinking thing.
What is more, Locke argued, it is likely (though not, technically, demonstrably certain) that this first eternal being is actually immaterial. Whether the original being were a single atom, or an eternal system of many parts, or whether every material thing thinks, Locke thought the possibility of a material thinking being at the start difficult to defend. On his view, we don't even have respectable grounds for supposing that a separate material reality is co-eternal with the necessarily cogitative first being. [Essay IV x 13-18] Locke's insistence on this point fits nicely with his frequent assertions that atheism amounts to nothing more than the supposition that matter is itself eternal, an objection to the views defended by Hobbes.
So God exists, but what is God? Here, as always, Locke depended upon the formation of ideas on empirical foundations. Although we have no direct experience of God, we do have ideas of reflection from experience of our own mental activity, and we have an idea of infinity as a simple mode derived from the idea of number, indefinitely expanded: put these together, and each of us develops an abstract complex idea of God as a being that possesses all cognitive abilities to an infinite degree. [Essay II xxiii 33-35] In principle, this idea differs from any of our other ideas of thinking things only in the infinity of the divine attributes. God is abolutely perfect in every conceivable respect, and this is ample provision for a deduction of the moral law.
Locke was also interested in traditional issues about the relation between faith (assent to revealed truth) and reason (discovery of demonstrative truth) as alternative sources of human conviction. For propositions about which the certainty of demonstrative knowledge is unavailable, our assent may be grounded upon faith in revelation; but Locke argued that the degree of our confidence in the truth of such a proposition can never exceed our assurance that the revelation is of genuinely divine origin, and this itself is subject to careful rational evaluation. [Essay IV xvi 14] Since God has provided both avenues of belief for the benefit of human achievement, Locke supposed, they can never conflict with each other if properly used. Faith is appropriate, but only with respect to vital issues that lie beyond the reach of reason; to allow any further extent to non-rational religious convictions would leave us at the mercy of foolish and harmful speculations. In any case where revelation (understood as an extraordinary communication from God) and ordinary human reason coincide in support of the same truth, Locke argued, it is reason that provides the superior ground, since our assurance of the reliability of the revelation itself can never exceed the perfection of demonstrative certainty. [Essay IV xviii 4-11]
Divorcing the (properly complementary) resources of revelation and reason, Locke supposed, is dangerous because it tends to encourage the promulgation of reckless claims of the revealed origin of otherwise incredible propositions. "Enthusiasm," as Locke and many of his contemporaries feared, rests solely upon the emotional strength of persuasion as grounds for assent, and this is formally independent of the objective likelihood of its purportedly divine origin.
[Essay IV xix 4-9]
In a sense, then, reason emerges from Locke's discussion as the ultimate arbiter of all legitimate human assent: either it discovers the demonstrative connections through which the truth of an individual proposition can be established with certainty, or it plays the most crucial role in certifying the legitimacy of a revealed proposition as divine rather than merely delusive.
[Essay IV xix 12-16]
Even though faith can play a role in human life, reason remains the most important basis for genuine human knowledge.