Witherspoon did not derive his politics from the Bible. He did not think the Christian God had a specific role to play in public life, where the rule of nature prevailed. And he did not worry about assuming an Enlightenment perspective on political matters.
This, however, is directly contrary to what Witherspoon himself tells us. In his sermon on the extent of visible religion, for instance, Witherspoon is very insistent that "the Christian God" has a "specific role to play in public life", since he holds the fairly common Christian view that religious practice includes all of one's moral life. (The phrase 'the Christian God' is telling since it seems to assume -- which Witherspoon certainly would not -- that you can take Christianity, or orthodox Christianity, as Rowe calls it, and isolate it into a few purely dogmatic beliefs, like that of the Trinity, separate from everything else.) Witherspoon argues that it is essential for Christians to show that no one can discharge his duty any public or social duty whatsoever so well as those who "are renewed in the spirit of their minds," because they, "having the love of God shed abroad in their hearts, must of consequence love their brethren also." Thus for Witherspoon any fulfillment of the duties of public life (he explicitly includes those pertaining to magistrates and rulers) must be, for the Christian, an expression of Christian life and love of God. Yes, the Christian God, if you want to use that odd pleonasm. He considers this essential because, as he says, reason and argument "is but an uninformed picture for the living man"; for people to be guided in what they must do, they need not merely speculative reasoning about the good, but, much more importantly, the sensible representation of the good as found in the life of the righteous. That he does get at least some of his politics from Scripture is seen by his footnote on the magistracy in his treatise criticizing stage-plays and the famous sermon on the dominion of providence over the passions of men.
Thus there really is no evidence that Witherspoon "attempted to synthesize Christianity with Enlightenment rationalism" as is suggested in the post; this comes, I think, from confusing what would have been standard education with "Enlightenment rationalism," however we are to understand that. After all, Witherspoon was Scottish; the Enlightenment he knew didn't need to be synthesized with Christianity, because it was a largely Christian movement. With the exception of a small circle around Hume, it was all Presbyterian: Hutcheson, Reid, Campbell, Beattie, etc. It is true that Witherspoon stands alone as a member of the Evangelical party of the Church of Scotland; the other major names were all members of the Moderate party of the Church, and thus to that extent his theological opponents. But, of course, both camps shared the fundamentals of the faith (while other differences occasionally arose, they differed mostly on the role of the people in church, the Evangelicals being the more democratic), and also pride in what was arguably the most successful education system in the period. There was no need for synthesis; it was already there -- enough so that one can perhaps argue that too many people accepted the particular way they were synthesized a little too uncritically. But people like Hume had to go to great lengths to break down this synthesis, and in the more freethinking Founders we find simply a (slightly) more advanced stage of this process (aided, no doubt, by the influence of the more successful freethinking movements in France).
