I've been doing some digging there too. I had known that JQA had started off, like his father, a Unitarian Congregationalist and thought that he had converted to Calvinism sometime in his early adult life. Indeed, one reason why I know of JQA's drift towards Calvinism is because father and son discussed religion in their correspondence and previously I reproduced a passage from a letter where the elder Adams discusses his Unitarianism and his son's Calvinism:
We Unitarians, one of whom I have had the Honour to be, for more than sixty Years, do not indulge our Malignity in profane Cursing and Swearing, against you Calvinists; one of whom I know not how long you have been. You and I, once saw Calvin and Arius, on the Plafond of the Cathedral of St. John the Second in Spain roasting in the Flames of Hell. We Unitarians do not delight in thinking that Plato and Cicero, Tacitus Quintilian Plyny and even Diderot, are sweltering under the scalding drops of divine Vengeance, for all Eternity.
John Adams to John Quincy Adams, March 28, 1816, Adams Papers (microfilm), reel 430, Library of Congress.
However, in doing further research it appears that John Quincy Adams's conversion wasn't complete, but that he vacillated between Unitarianism and Calvinism throughout the rest of his life. Or at least, that's the story that I get from these two links.