In very similar words, and only a few years later, the English philosopher John Locke argued that, since men differ in their interpretations of the Bible and always will, none should seek to impose his opinions on another, and that their differences should not divide them. In his first _Letter Concerning Toleration_ (1689), Locke wrote:
Since men are so solicitous about the true church, I would only ask them here by the way, if it be not more agreeable to the Church of Christ to make the conditions of her communion consist in such things, and such things only, as the Holy Spirit has in the Holy Scriptures declared, in express words, to be necessary to salvation?
And Rupertius Meldenius made the classic statement of this principle when he said: "In essentials, unity; in non-essentials, liberty; in all things, charity."
But the churches did not respond to this appeal for liberty of opinion within the church that there might be union of Christians in one church. Slowly, however, the governments of most European countries in which the Roman Catholic Church did not exercise control yielded to the demand for liberty of religious opinion within the state. With this grant of toleration to churches which were mutually intolerant, the states preserved their unity, while the church sank into a condition of complacent sectarianism. During the seventeenth century there had been many pleas for church unity through liberty. The eighteenth century thought much about liberty and little about unity. But it is to be remembered that, when a new call to unity was sounded in America at the beginning of the nineteenth century, it was the renewal of a campaign that already had a long history. It came at a time when the churches in America, happy in the complete liberty they enjoyed and in their freedom from state control and equality before the law, had ceased to be much concerned about unity and had settled into the conviction that division and denominationalism represented the normal condition of the church.
The other principle stressed by "the reformation of the nineteenth century" was the restoration of primitive Christianity. That also had a long history, which can be only sketched. Thomas and Alexander Campbell made a new use of this idea, and it will have a large place in the story of their work, but in order to understand their contribution it is necessary to note that the idea itself was not new. The oldest Christian bodies claim to have preserved primitive Christianity uncorrupted, and every reforming movement in the history of the church has claimed in some sense to offer a restoration of its pristine purity. A few citations, among many that might be offered, will make this clear.