Final answer:
Thomas Jefferson was deeply concerned about the Missouri Compromise, viewing it as a temporary fix that failed to resolve the underlying conflict over slavery, which would continue to deepen divisions and threaten the Union.
Step-by-step explanation:
Thomas Jefferson's reaction to the Missouri Compromise was one of deep concern for the future unity of the United States. When he said the Compromise was "like a firebell in the night" that filled him with terror and considered it "the knell of the Union," he meant that the agreement was merely a temporary solution to the growing sectional conflict over slavery and not a lasting resolution.
The phrase "a reprieve only, not a final sentence" implies that Jefferson saw the Compromise as just a temporary delay of the inevitable conflict that the issue of slavery would continue to fuel, potentially leading to disunion. He recognized that the establishment of a geographical line for slavery might quiet the issue for a time but would also create a permanent divide that would be deepened with every subsequent controversy over slavery, eventually threatening the survival of the Union itself.