Although Locke remained in the Church of England all his life, his religious thinking was radical. His library contained the writings of Castellio, Socinus, Biddle, the Racovian Catechism, and most of the influential Socinians. He believed in a simple, reasonable, benevolent Christianity based on the Bible and especially on the moral teachings of Jesus. For him, Scripture showed a clear way to eternal life led by Christ, "the Captain of our salvation."

When, therefore, the leading churchmen abandoned Christian truth for subtle speculations and rancorous hair-splitting during the Trinitarian Controversy, the cause of religion suffered in his eyes. In an attempt to restate its essential doctrines, Locke in 1695 anonymously published a treatise on The Reasonableness of Christianity. One minister called it "one of the best books that have been published for at least these sixteen hundred years." Appearing at the height of the Trinitarian Controversy, it strengthened the liberal cause and placed Locke, the leading philosopher of his time, in the same camp theologically with the Socinians and Unitarians.

Though the works of Nature, in every part of them, sufficiently evidence a Deity; yet the world made so little use of their Reason, that they saw him not, where even by the impressions of himself he was easy to be found. Sense and lust blinded their minds in some, and a careless inadvertency in others, and fearful apprehensions in most (who either believed there were, or could not but suspect there might be, superior unknown Beings) gave them up into the hands of their priests, to fill their heads with false notions of the Deity, and their worship with foolish rites, as they pleased: and what dread or craft once began, devotion soon made sacred, and religion immutable. In this state of darkness and ignorance of the true God, vice and superstition held the world. Nor could any help be had, or hoped for from Reason; which could not be heard, and was judged to have nothing to do in the case: the priests, every where, to secure their Empire, having excluded Reason from having any thing to do in religion. And in the crowd of wrong notions, and invented rites, the world had almost lost the sight of the one only true God. The rational and thinking part of mankind, 'tis true7 when they sought after him, found the one supreme, invisible God: but if they acknowledged and worshipped him, it was only in their own minds. They kept this truth locked up in their own breasts as a secret, nor ever dared venture it amongst the people, much less amongst the priests, those wary guardians of their own creeds and profitable inventions. Hence we see that Reason, speaking never so clearly to the wise and virtuous, had never authority enough to prevail on the multitude, and to persuade the societies of men, that there was but one God, that alone was to be owned and worshipped. The belief and worship of one God, was the national religion of the Israelites alone: and if we will consider it, it was introduced and supported amongst the people by Revelation. They were in Goshen, and had light, whilst the rest of the world were in almost Egyptian darkness, without God in the world. . .

In this state of darkness and error, in reference to the True God, our Saviour found the world. But the clear Revelation he brought with him, dissipated this darkness; made the One Invisible True God known to the world: and that with such evidence and energy, that Polytheism and Idolatry hath no where been able to withstand it: but wherever the preaching of the truth he delivered, and the light of the Gospel hath come, those mists have been dispelled. And in effect we see that since our Savior's time, the Belief of One God has prevailed and spread it self over the face of the Earth…

God out of the infiniteness of his mercy, has dealt with man as a compassionate and tender Father. He gave him Reason, and with it a Law: that could not be otherwise than what Reason should dictate; unless we should think, that a reasonable Creature should have an unreasonable Law. But considering the frailty of man, apt to run into corruption and misery, he promised a Deliverer, whom in his good time he sent; and then declared to all mankind, that whoever would believe him to be the Savior promised, and take him now raised from the dead, and constituted the Lord and Judge of all men, to be their king and ruler, should be saved. This is a plain intelligible proposition; and the all-merciful God seems herein to have consulted the poor of this world, and the bulk of mankind. These are articles that the laboring and illiterate man may comprehend. This is a religion suited to vulgar capacities; and the state of mankind in this world, destined to labor and travail. The writers and wranglers in religion fill it with niceties, and dress it up with notions, which they make necessary and fundamental parts of it; as if there were no way into the Church, but through the Academy or Lyceum. The greatest part of mankind have not leisure for learning and logic, and superfine distinctions from the schools. Where the hand is used to the plough and the spade, the head is seldom elevated to sublime notions, or exercised in mysterious reasoning. 'Tis well if men of that rank (to say nothing of the other sex) can comprehend plain propositions, and a short reasoning about things familiar to their minds, and nearly allied to their daily experience. Go beyond this, and you amaze the greatest part of mankind: and may as well talk Arabic to a poor day laborer, as the notions and language that the books and disputes of religion are filled with;> and as soon you will be under-stood. . . . Had God intended that none but the learned scribe, the disputer or wise of this world, should be Christians, or be saved, thus religion should have been prepared for them, filled with speculations and niceties, obscure terms and abstract notions. But men of that expectation, men furnished with such acquisitions, the Apostle tells us, I. Cor. i. are rather shut out from the simplicity of the Gospel; to make way for those poor, ignorant, illiterate, who heard and believed promises of a Deliverer, and believed Jesus to be him; who could conceive of a man dead and made alive again, and believe that he should at the end of the world, come again and pass sentence on all men, according to their deeds. That the poor had the Gospel preached to them; Christ makes a mark as well as business of his mission, Mat. xi. 5. And if the poor had the Gospel preached to them, it was, without doubt, such a Gospel as the poor could understand, plain and intelligible: and so it was, as we have seen, in the preachings of Christ and his Apostles.