Do we know with any certainty if (any founding fathers) read Spinoza?

Great Question. I've been thinking about it ever since I first saw it. I hope I can do it justice. .

Short Answer

Directly I only found evidence of Jefferson, Adams, and Franklin. Indirectly Spinoza's fingerprints are all over the American Declaration of Independence and Constitution and highly influential of all the Founding fathers I investigated.

I investigated thought leaders of the American revolution, and a few founding fathers who I thought fit the profile iof who might have known more Enlightenment scholars from continental Europe, beyond the English school.

Among these thought leaders of the American Enlightenment were

• John Adams, - direct link.
• Thomas Jefferson - references Spinoza in letter to Adams, book in his library
• James Madison, - influenced shown in Federalist Papers, Constitution
• George Mason, - influenced
• James Wilson, - influenced
• Ethan Allen, - Diest, influenced
• John Jay. - influenced shown in Federalist Papers
• Alexander Hamilton, - influenced shown in Federalist Papers
• Benjamin Franklin, - (*)Had english copy of Tractatus in his library. Claimed was used as reference during Constitution debate.
• George Washington, thrown in largely because I love the man. demonstrated Influence.
• James Monroe, - Diest, influenced

(*) Franklin is the only founding father who signed all four of the foundational documents of the United States.

• the Declaration of Independence (1776),
• the Treaty of Alliance with France (1778),
• the Treaty of Paris establishing peace with Great Britain (1783)
• the U.S. Constitution (1787).

I found evidence Jefferson, Adams, and Franklin where familiar with Spinoza directly, but only Jefferson refereed to him in a correspondent with Adams after both had left office. Franklin like Jefferson had a book by Spinoza in his library, which is said to have been consulted as the Constitution was debated, but I could only find one reference claiming that. It is perhaps no coincidence that all three of these founding fathers had spent significant time in France, and all three were non-traditional religiously (unitarians or deists). Given Spinoza had three times been investigated by heretical bodies.. ( Jewish, Catholic, and Protestant) I thought that might be pertinent.

It seems that while Spinoza's influences and fingerprints are all over the declaration of independence, and Constitution, in important ways; most of the founding fathers could have been unaware of him. The sited influences for the founding fathers were mostly from the English school of enlightenment, and Spinoza was an influential forerunner of that movement. So perhaps his fingerprints are on our founding fathers, simply because his fingerprints were on the entire Age of Enlightenment, or perhaps his lack of citation was an after effects of the controversy which followed Spinoza in life, ( jewish, religious heritic ).

Finally I found a quote which attributes the lack of citation of Spinoza from the founding fathers, as an example of how provincial the United States was in the mid 1700's.

Religion, State and Society: Jefferson's Wall of Separation in Comparative Perspective.
By R. Ramazani, Robert Fatton.
I note in passing that there seems to be little evidence that American thinkers read or had any serious knowledge of Spinoza - or that at least is the impression I retain after consulting such authorities as Bernard Bailyn, Noah Feldman, Robert MiddleKauff, and James Turner on this question. That observation seems both unsurprising, given Spinoza's background and status, yet also noteworthy as a comment upon the provincialism of eighteenth century American Thinking.

Perhaps a factor of how limited and rare different texts and ideas were that reached the Americas. This brings me back to the Founders I did identify, all having spent considerable time in Europe namely France. I then went back to James Monroe, because he fit the mold of the previous founders identified. An educated (William and Marry same as Jefferson) deist who had spent considerable time in France as Ambassador after Jefferson, Adams and Franklin. But I could find nothing linking Monroe with Spinoza, other than the indirect. Monroe vouched for Thomas Paine and helped him get out of France's revolutionary prison. Paine being an important central author of Enlightenment ideas for some of the Founding Fathers, and a friend of both Jefferson and Monroes.

More Detailed Answer

Background:.
Enlightenment philosophy revolved around reason as the primary source of societal authority and legitimacy. The challenge faced by it's practitioners was to find the different methods and approaches to apply reason to various broad topics. This was contrasted by pre enlightenment era's which were dominated by dogma, primarily all encompassing religious dogma (Catholic Church) which legitimacy and authority came from faith and mastery/interpretations of articles of faith.

Spinoza's:.
Baruch Spinoza(1632-1677) lived at the end of the Protestant Reformation(1517 – 1648) and was a foundational influence on the European Age of Enlightenment(1685-1815). The European Age of Enlightenment was the inspiration and bases of the American Enlightenment period (1714–1818) which encompassed all the American founding fathers.

American Founding Fathers If we limit ourselves to sourced material relied upon directly, by the founding fathers it appears they read works mostly of British origin(English, Scottish, Irish). The most famous was the English philosopher John Locke. Other influences(also mostly British)include:

• Henry St. John, Viscount Bolingbroke (1678-1751) English Politician

  Major influence on the American patriots John Adams, Thomas Jefferson and James Madison. Adams said that he had read all of Bolingbroke's works at least five times; indeed, Bolingbroke's works were widely read in the American colonies, where they helped provide the foundation for the emerging nation's devotion to republicanism in the United States. His vision of history as cycles of birth, growth, decline and death of a republic was influential in the colonies,2 as was his contention on liberty: that one is "free not from the law, but by the law"

• Algernon Sidney(1623-1683) - English Politician, Political Philosopher.

  among the greatest influences on the Founders along with John Locke.

• Alexander Pope (1688-1744) - Famous influential English Poet.

  An Essay on Man (1733). All are but parts of one stupendous whole,
  Whose body nature is, and God the soul ...
  All nature is but art, unknown to thee;
  All chance, direction, which thou cannot see.
  All discord, harmony not understood;
  All partial evil, universal good
  And, spite of pride, in erring reason's spite,
  One truth is clear: Whatever is, is right.

• Anthony Ashley Cooper 4th Earl of Shaftesbury(1671-1713) English Philosopher.

  Shaftesbury was a pronounced liberal and very much opposed to religious intolerance and persecution. The Fundamental Constitutions of Carolina, the laws for the new province, were the work of Shaftesbury's friend and secretary, the philosopher John Locke, but they contain evidences of Shaftesbury's collaboration, too. The laws he helped to write produced the greatest measure of political and religious freedom in British North America (and, indeed, in much of the world). He was the author of the Habeas Corpus Act whereby an accused man cannot be held indefinitely in prison without trial, an English law which was passed along later to the United States of America.

  Shaftesbury not only had his holdings in Carolina, but he had been part owner of a sugar plantation on Barbados, and a shareholder in the Hudson Bay Company.

• Thomas Gordon (169?-1750) - Scottish writer

• John Trenchard (1662-1723) - English writer

  Thomas Gordon and John Trenchard wrote "Cato's Letters" 1720-23, 144 essays "Long regarded as a keynote text in understanding the ideological foundations of the American Revolutionary generation." historian Clinton Rossiter stated "no one can spend any time on the newspapers, library inventories, and pamphlets of colonial America without realizing that Cato's Letters rather than John Locke's Civil Government was the most popular, quotable, esteemed source for political ideas in the colonial period."

• Anthony Collins (1676 – 1729 ) English Philosopher Diest.

  Deism and the Founding of the United States [It was this English tradition that influenced leading American deists, such as Thomas Jefferson, James Madison, and Benjamin Franklin; and when Thomas Paine published his notorious defense of deism, The Age of Reason (published in two parts, 1794, 1795; with a third part—a criticism of biblical prophecies—added in 1807), he drew heavily from his English predecessors, such as Anthony Collins.

• John Toland (1670–1722). Irish-born Philosopher

Of his influence, humanities professor Robert Pattison wrote: "Two centuries earlier the establishment would have burned him as a heretic; two centuries later it would have made him a professor of comparative religion in a California university. In the rational Protestant climate of early 18th-century Britain, he was merely ignored to death."

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However, Toland managed to find success after his death: Thomas Hollis(1720–1774), who published his works and sent collections of Enlightenment Thinkers including Toland to America including generous donations to Harvard College.

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Spinoza's largest influence on the American Revolution was as a foundational and hugely influential figure of the English Enlightenment Philosophers.

Jefferson references Spinoza in a Letter to John Adams April 11, 1823

Now one sixth of mankind only are supposed to be Christians: the other five sixths then, who do not believe in the Jewish and Christian revelation, are without a knoledge of the existence of a god! This gives completely a gain de cause to the disciples of Ocellus, Timaeus, Spinosa, Diderot and D'Holbach. The argument which they rest on as triumphant and unanswerable is that, in every hypothesis of Cosmogony you must admit an eternal pre-existence of something; and according to the rule of sound philosophy, you are never to employ two principles to solve a difficulty when one will suffice.

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James Madison - Memorial and Remonstrance Against Religious Assessments

“all men are equally entitled to the free exercise of religion” — to emphasize that religious liberty is an unalienable natural right and not the gift of society, government, or any other person.

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James Madison, Alexander Hamilton nad John Jay, Federalist Papers 51

"But What is government itself but the greatest of all reflections on human nature? If men were angels, no government would be necessary"

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Benjamin Franklin.
Spinoza, by Richard H. Popkin A copy of the Tractatus in English was in the library of Benjamin Franklin, and was used as a resource by those taking part in the constitutional debates of 1776. I have personally examined the copy from Frankin's library and unfortunately there are no markings and no indication as to who might have read it.

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George Washington Letter to the Hebrew Congregation at Newport. Upon his election as President, many churches, congregations and religious societies wrote to George Washington to congratulate him on his new office, and he replied to each of them with personalized messages of thanks for their well-wishes. In this reply to the Hebrew Congregation of Newport, Washington applauded the people of the United States for rejecting the European practice of religious “toleration,” embracing instead the “enlarged and liberal policy” that religious liberty is a natural right — and not a gift of government — which all citizens are equally free to exercise.

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Declaration of Independence:. Jefferson

When, in the course of human events, it becomes necessary for one people to dissolve the political bonds which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the laws of nature and of nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.

"Laws of nature" and "nature's God" are direct quotes from Spinoza. Also the tone of the Declaration makes no reference to revelation or mysticism, but rather to ethical truths that can be discovered exclusively through human reason. These too are Spinoza's infl

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Spinoza Experienced religious intolerance from many different communities. On September 14, 2004 James Carroll wrote for the Boston Globe:

Spinoza was himself expelled from the Synagogue (1656), investigated by the Catholic Inquisition (1659), and banned by the Calvinist Synod (1670). This experience of omnidirectional religious intolerance underwrote his two-fold new idea -- that the state's first obligation is to protect the freedom of conscience of each citizen; to do so the state must not itself be religiously identified.

Spinoza's "Theologico-Political Treatise", contained the idea that no human institution was above Criticism. He advocated for multiple structure which would play off of one another to give a more dependable desired path.

The United States Constitution with our Checks and Balances, and Religious Freedom based upon the Governments prohibition on religious sides are all ideas first expressed by Spinoza.