Here is a taste from Part 1:
But what are the essentials? Specifically, what are the essential teachings which one must accept to be a Christian? Many have a rather expansive view of those. But Locke suspected they had inflated something simpler. In the winter of 1694-5, he decided to be a good Protestant and to go back to the sources. What does the New Testament, he wondered, demand of us, as far as beliefs are concerned? Does it require, for instance, believing “grace” as taught by Calvinists? Or the contents of the “Athanasian” creed about the Trinity and the two natures of Jesus? The simplified but vague “deity of Christ” so insisted upon by present-day evangelical Protestants?
This relates to the study of the American Founding in the sense of whether the key Founders were "Christians." Under a more generous standard -- one that could, for instance rope Mormons who believe Jesus is the Messiah in -- the key Founders including arguably Jefferson were "Christians." Under stricter standards, like those conservative evangelicals tend to hold, the key Founders weren't "Christians" but something else.Locke examined this question, and found an explicit answer in scripture. All that Christians must believed, he argues, can be summarized like this: Jesus is the Messiah.