When discussing the Old Testament, Adams ranks which version he prefers; though he notes he thought all translations contained errors. As he wrote to his father on July 7th, 1814:
I promised you that I would answer your questions of my opinions with regard to the Bible, and of my acquaintance with it—I have not studied the Canon of the Old Testament, because to my deep and constant regret I do not understand the languages in which it was written—I have never learnt either the Hebrew, or Chaldaic Characters, and therefore never could read a line of the Old Testament, in the Original—I have only read it in the Modern English French and German Translations for I have hitherto not even had the opportunity of going through either the Greek Septuagint or the Latin Vulgate, as I hope at some future day to do—of the translations which I have read, that in German, made by Luther, is incomparably the Best—The French one, originally made by Calvin, and revised by the Pastors of the Church at Geneva, is upon the whole not quite equal to the common English Bible published with the Dedication to James the 1st.—But in all there are a multitude of errors; and they are all so far from giving me satisfaction, that I shall never forgive myself, for neglecting to learn the Hebrew, when the opportunity for learning it was in my own hands.
So as we see, Adams puts the KJV above Calvin's Geneva Bible, but also puts Luther's German translation at the very top. He notes that because he didn't read Hebrew that he's not competent in the original language to fully comprehend the Old Testament.
The elder John Adams adhered to an extremely "heterodox" faith that contrasted to the orthodoxy that was more institutionally ingrained in late 18th century America. He considered himself a "liberal unitarian Christian." The scholar Dr. Joseph Waligore terms him a "Jesus-Centered-Deist." Whereas Dr. Gregg Frazer terms him a "theistic rationalist."
These terms are used to distinguish from that more conventional orthodox Trinitarian Christianity. John Quincy Adams, during a period of time in his adult life, opted for that more traditional understanding of the faith, with a Calvinist bent.
However, JQA seemed to vacillate, during his adult life, between the two -- the more conventional Calvinistic Christianity and his father's heterodoxy. I've gotten confused more than once when reading the younger Adams during periods of time in history when he supposedly was a traditional Calvinist, and it sounds like it's his father's heterodoxy talking.
As the saying goes, the apple doesn't fall far from the tree.