Anti-Roman Catholic bigotry tied the "Protestant Christians" in America during the Founding era and for some time thereafter. This passage by the "Protestant Christian" Jonathan Mayhew in 1759 on Roman Catholicism & Quebec is instructive of the anti-Roman Catholic zeitgeist:
Dost thou not know, that those who fight for a Tyrant, will not fight like free-born Britons ? Perhaps thou thinketh thyself again at Ticonderoga—But dost thou not see, who it is at the head of that little veteran army, by his presence infusing courage enough into each breast, to make every man a hero ? Or, perhaps, thou thinkest thy relicks, thy crosses, and thy saints, either St. Peter, or thy great Lady, whom thou profanely stilest "The mother God," will now befriend, and make thee victorious. But remember, that little host now in array against thee, worship the God that made the heavens, earth, and seas, with all that they contain; the Lord of hosts is his name! His is the glory and the victory ; and know, that the event of this battle shall be accordingly! Cross thyself speedily, if thou thinkest it will be of any advantage to thee! Mercy to thy soul, notwithstanding violated faith at Lake George, once St. Sacrament! But alas! be assured, that yonder gloomy wood on thy right, affords not laurels, but cypress for thy brows!
I put "Protestant Christian" in quotation marks when describing Mayhew because he was a unitarian, that is someone who thought himself a "Christian" but was unorthodox.
It's ironic that orthodox Trinitarian Protestants might have found common ground with unitarians of the Founding era through anti-Roman Catholic bigotry. Given their shared common ground in historic orthodox Trinitarianism, arguably Trinitarian Protestants and Roman Catholics have more in common with one another, theologically, than either do with the "unitarianism" of Mayhew and many of America's key Founders and the other philosophers and divines who influenced them.
Yet, to many Catholics, Protestants like Mayhew and Trinitarian Calvinists are indeed in the same box: They are unorthodox heretics, deviating from the one true Church's official teachings.
In fact, I came across one such Roman Catholic, a very intelligent and accomplished American attorney, recently. He wrote:
For Catholic purposes, “orthodox” means accepting all of the Church’s teachings. One can accept the Nicene Creed but still be a heretic. ... But even assuming [Calvin accepted the Nicene Creed], it’s not enough, since he also (as I recall) denied the doctrine of free will, the communion of saints, and the sacrifice present in the Mass, which are fairly important doctrines. You and the modern theologians (whom I assume are Protestants) are free to use the term “orthodox Christianity” to include “views formally condemned by the Church as heretical and therefore not orthodox,” but that’s not the sense in which I’m using it.