It was thought to be treasonous.
"No man, no association of men, no state or set of states has a right to withdraw itself from this Union, of its own accord. The same power which knit us together, can only unknit. The same formality, which forged the links of the Union, is necessary to dissolve it. The majority of States which form the Union must consent to the withdrawal of any one branch of it. Until that consent has been obtained, any attempt to dissolve the Union, or obstruct the efficacy of its constitutional laws, is Treason--Treason to all intents and purposes. . . . This illustrious Union, which has been cemented by the blood of our forefathers, the pride of America and the wonder of the world must not be tamely sacrificed to the heated brains or the aspiring hearts of a few malcontents. The Union must be saved, when any one shall dare to assail it." [Richmond Enquirer, 1 November 1814--The Enquirer, by the way, was Thomas Jefferson's favorite newspaper).
You might also look into what happened to the Federalist party and the accusations that drove them to extinction.
Interesting snippet ... what was the context that spurred such a discussion back in 1814?
After I posted I realized that what I meant to post was "Generally, secession was not thought to be unconstitutional until the concept was challenged by the Lincoln administration." My bad!
It was thought to be treasonous. "No man, no association of men, no state or set of states has a right to withdraw itself from this Union, of its own accord. "
One newspaper's opinion is still only one person's opinion, and we all know about opinions, ie - that everyone has one...... I believe that most folks held the opposite opinion.