There Franklin says:
I agreed with you in Sentiments concerning the Old Testament, and thought the Clause in our Constitution, which required the Members of Assembly to declare their belief that the whole of it was given by divine Inspiration, had better have been omitted. That I had opposed the Clause but being overpower'd by Numbers, and fearing might in future times be grafted on [it, I Pre]vailed to have the additional Clause that no [further or more ex]tended Profession of Faith should ever [be exacted. I ob]serv'd to you, too, that the Evil of it was [the less, as no In]habitant, nor any Officer of Government except the Members of Assembly, were oblig'd to make that Declaration. So much for that Letter. To which I may now add, that the[re are] several Things in the old Testament impossible to be given by divine Inspiration, such as the Approbation ascrib'd to the Angel of the Lord, of that abominably wicked and detestable Action of Jael the Wife of Heber the Kenite. If the rest of the Book were like that, I should rather suppose it given by Inspiration from another Quarter, and renounce the whole.During the time in which they were establishing religious liberty, heterodox freethinkers like Franklin and the other key Founders felt comfortable sharing their religious secrets with certain trustworthy friends. As such, these "heretics" tended to converse amongst themselves. So it shouldn't surprise that John Calder turned out to share similar religious convictions as Franklin.
I want to again thank Bill Fortenberry for turning my attention to something important: this time it was John Calder's letter to which Franklin responded.
Here is a big taste:
I now enter unwillingly on a subject so insignificant, but I must necessarily say something of myself, as an apology for what it might else be impertinent in me to mention. On the dissolution of the Religious Society of which Mr. Radcliffe and I were the Ministers, which happened soon after you left England I declined the stated exercise of the profession to which I was educated, and have ever since been a private member of the Church of Unitarian Christians in Essex Street at the opening of which you was present. There only I sometimes officiate occasionally as Minister and never but when necessity requires it. In the mean while, in a comfortable retirement about a mile from town, my books have been my principal companions, and the culture of a garden my chief amusement. Here I have for some years inwardly cheriched the hopes of seeing you again and endeavoured to save all I can, to transport me and my companions to Pennsylvania, where whether I accompany them or not I mean they shall be ultimately deposited in the Library of which you was the founder. Turned as I am the of life, being but a year younger than your very good friends and mine Dr. Priestley and Mr. Lee and urged by no grievous necessities nor unfavourable prospects here, perhaps even the Friends I mention will condemn my resolutions. But with such undisclosed views I have long secretly sighed for a sight of the American Constitutions and have been within these few days in possession of my wishes. I concern myself chiefly with the Constitution of the state in which my views terminate, and I rejoice that it hav in all respects the preheminence. In its Council of Censors there is a resource for the removal of the objection, for I have but one, and therefore after what I have said, I know you will forgive my taking the liberty of mentioning it on the way of query.
Is the last clause of the Declaration in Sect. 10 of Chap. II reconcileable to the clause of the 2d Article of the Declaration of Rights which says, “Nor can any man who acknowledges the Being of a God be justly deprived of any civil right as a Citizen on account of his religious sentiments, or peculair mode of religious worship.” I cannot think that the State of Pennsylvania would have even endangered its welfare by admitting freely and universally to a denizonship in it, “all foreigners of good character” Christians or not Christians.
But passing from this, there are Christians and sincere worthy Christians who after all their pains to make up their minds on the subject of the divine inspiration of the Old Testament especially, must express themselves as our friend Mr. Lee did on another subject, when he said I have been a great part of my life, endeavouring to understand it, but I cannot yet tell you what a Libel is. If the State of Pennsylvania wishes to grant citizenship to all foreigners of good character who are Christians, why establish a declaration which some Christian foreigners of good character must object to? Is it an incredible thing that a man be really a Christian, who is not yet a Jew? Or is it indispensibly requisite that a man must first be a jew before he can be qualified to be a good of the State of Pensylvania? May not the Friends of Christianity have connected it injudiciously, and injured its cause by connecting it more closely with Judaism than its Author and first publishers did? Are not the Evidences of Christianity and the evidences of Judaism destinct? Why then complicate them with each other so odd as that they must necessarily stand or fall together?
This is what the site says about Calder:
Member of the Club of Honest Whigs, of which Franklin was also a member.
Employed by the Duke of Northumberland as his private secretary at Alnwick Castle and in London. In charge of the private library bequeathed by Dr. Williams to nonconforming clergy.
Assistant to Ebenezer Radcliffe, pastor of the Presbyterian congregation in Aldgate.
After the congregation was dissolved in 1774, he declined to exercise his ministerial functions and devoted himself to writing.
A private member of the Church of Unitarian Christians.
Born in Aberdeen, Scotland.
The Club of Honest Whigs was disproportionately comprised of unitarians. That's why when Franklin told Ezra Stiles of his creed, he said, "I have with most of the present Dissenters in England, some Doubts as to [Jesus'] Divinity..." [Bold Face Mine]. Franklin, no doubt, was referring to his unitarian cohorts in the Club of Honest Whigs.
In researching Calder, I also found this wiki on the Society for Promoting the Knowledge of the Scriptures. The site says this "was a group founded in 1783 in London, with a definite but rather constrained plan for Biblical interpretation. While in practical terms it was mainly concerned with promoting Unitarian views, it was broadly based." Calder was affiliated with it along with many others, including Joseph Priestley and Richard Price. Their membership overlapped with that of the Club of Honest Whigs.
This political theological worldview, whatever we term it, captured the minds of many "key Founders" like Ben Franklin.