He made the Declaration of Independence into a statement on the issue of slavery instead of what it actually was, which was an assertion that states had a right to be free and independent of a larger government which they saw as oppressing them.
He flipped the meaning almost 180 degrees, and used it to justify the very opposite thing which the Declaration said.
“He (Lincoln) flipped the meaning almost 180 degrees, and used it to justify the very opposite thing which the Declaration said.”
The Declaration of Independence remains the strongest brief ever written in support of secession. Lincoln, as you say, turned it into its opposite.