For, if strength of persuasion be the light which must guide us; I ask how shall any one distinguish between the delusions of Satan, and the inspirations of the Holy Ghost? He can transform himself into an angel of light. And they who are led by this Son of the Morning are as fully satisfied of the illumination, i.e. are as strongly persuaded that they are enlightened by the Spirit of God as any one who is so: they acquiesce and rejoice in it, are actuated by it: and nobody can be more sure, nor more in the right (if their own belief may be judge) than they.
We must not ask how strongly a proposition is believed, for the strength of the belief in no way correlates to the probability of its being true; rather, we must how rational a belief is:
Reason must be our last judge and guide in everything. I do not mean that we must consult reason, and examine whether a proposition revealed from God can be made out by natural principles, and if it cannot, that then we may reject it: but consult it we must, and by it examine whether it be a revelation from God or no: and if reason finds it to be revealed from God, reason then declares for it as much as for any other truth, and makes it one of her dictates. Every conceit that thoroughly warms our fancies must pass for an inspiration, if there be nothing but the strength of our persuasions, whereby to judge of our persuasions: if reason must not examine their truth by something extrinsic to the persuasions themselves, inspirations and delusions, truth and falsehood, will have the same measure, and will not be possible to be distinguished.
In the above paragraph, Locke is writing that reason will not tell us if a proposition is true, but reason will tell us if it is from God; and if it is from God, then it is true. A slightly different method for judging the rationality of a proposition appears a few pages later in his Essay Concerning Human Understanding:
But it is not the strength of our private persuasion within ourselves, that can warrant it to be a light or motion from heaven: nothing can do that but the written Word of God without us, or that standard of reason which is common to us with all men. Where reason or Scripture is express for any opinion or action, we may receive it as of divine authority: but it is not the strength of our own persuasions which can by itself give it that stamp. The bent of our own minds may favor it as much as we please: that may show it to be a fondling of our own, but will by no means prove it to be an offspring of heaven, and of divine original.
Reason and revealed text are here shown to be in harmony; they form a single continuous decision procedure for determining the truth-value of a given proposition. In this, Locke rejects the mere claim of a person who may claim to have received a supernatural revelation from God - such a claim carries no value in itself:
In what I have said I am far from denying, that God can, or does, sometimes enlighten men's minds in the apprehending of certain truths or excite them to good actions, by the immediate influence and assistance of the Holy Spirit, without any extraordinary signs accompanying it. But in such cases too we have reason and Scripture; unerring rules to know whether it be from God or no. Where the truth embraced is consonant to the revelation in the written word of God, or the action conformable to the dictates of right reason or holy writ, we may be assured that we run no risk in entertaining it as such: because, though perhaps it be not an immediate revelation from God, extraordinarily operating on our minds, yet we are sure it is warranted by that revelation which he has given us of truth.
Locke builds a case for the notion that to be human, and to be rational, grants access to truth - equal access. From this theorem, it is possible to move on to logical arguments for majority rule, and for the proposition that the consent of the governed is the source of a government's legitimacy. If we deny Locke's epistemology, we are obliged to deny his politics; if we reject his understanding of how humans access truth, we embrace dictatorship and governments indifferent to their subjects.