The background to this is that in 1819, a document surfaced in Mecklenburg County, North Carolina, claiming to have been written in 1775 that contained language very similar to the Declaration of Independence. It caused people to question whether Jefferson had actually written the Declaration of whether he had copied what he read from the Mecklenburg Resolves. It’s almost universally agreed upon today that the documents from Mecklenburg are a forgery, and at the time Jefferson tried to reassure Adams.
"But who can be the Demon to invent such a machine after five and forty years, and what could be his motive? Was it to bring a charge of Plagiarism against the Congress in 76, or against you, the undoubted, acknowledged draughtsman of the Declaration of Independence? Or could it be the mere vanity of producing a jeu d’esprit, to set the world a gasp and afford a topic of Conversation in this piping time of Peace?"
- John Adams to Thomas Jefferson, July 21, 1819
"Either these resolutions are a plagiarism from Mr. Jefferson’s Declaration of Independence, or Mr. Jefferson’s Declaration of Independence is a plagiarism from those resolutions. I could as soon believe that the dozen flowers of the Hydrangea, now before my eyes, were the work of chance, as that the Mecklenburg resolutions and Mr. Jefferson’s Declaration were not derived the one from the other."
- John Adams to William Bentley, August 21, 1819
To be fair, though he was certainly motivated by vanity, Adams was less insistent on believing Jefferson was a plagiarist than he was disturbed that people connected the whole Revolution to Jefferson and his Declaration, at the expense of everyone else.