Lincoln's speech to Congress- special session - July 4, 1861.
"It might seem at first thought to be of little difference whether the present movement at the South be called "secession" or "rebellion." The movers, however, well understand the difference. At the beginning they knew they could never raise their treason to any respectable magnitude by any name which implies violation of law. They knew their people possessed as much of moral sense, as much of devotion to law and order, and as much pride in and reverence for the history and Government of their common country as any other civilized and patriotic people. They knew they could make no advancement directly in the teeth of these strong and noble sentiments. Accordingly, they commenced by an insidious debauching of the public mind. They invented an ingenious sophism, which, if conceded, was followed by perfectly logical steps through all the incidents to the complete destruction of the Union. The sophism itself is that any State of the Union may consistently with the National Constitution, and therefore lawfully and peacefully, withdraw from the Union without the consent of the Union or of any other State. The little disguise that the supposed right is to be exercised only for just cause, themselves to be the sole judge of its justice, is too thin to merit any notice.
With rebellion thus sugar coated they have been drugging the public mind of their section for more than thirty years, and until at length they have brought many good men to a willingness to take up arms against the Government the day after some assemblage of men have enacted the farcical pretense of taking their State out of the Union who could have been brought to no such thing the day before..."
No mention of slavery anywhere in the message. No mention of the intent to protect slavery as a cause of secession. He mentions an "ingenious sophism" or deceiving argument and even only points to states rights to secede as the question. He mentions a "insidious debauching" of the mind. He gives no detail to what methods were used, the reader would have had to know what he meant.
Lincoln is stating the methods and arguments used by the leaders of Southern secession were meant to deceive the Southern public into agreeing with the secession movement. In other words, all arguments, even the argument to secede to protect slavery was really a lie.