Answer:
(This is the sample answer so you would want to re-word it!)
Lincoln begins by trying to convince Southerners that they shouldn’t be worried about his presidency. The main idea of Lincoln’s speech, however, seems to be the constitutionality of the secession of the Southern states. Most of his speech is spent discussing this topic. This excerpt is a good example:
I hold that, in contemplation of universal law and of the Constitution, the Union of these States is perpetual. Perpetuity is implied, if not expressed, in the fundamental law of all national governments. It is safe to assert that no government proper ever had a provision in its organic law for its own termination. Continue to execute all the express provisions of our National Constitution, and the Union will endure forever—it being impossible to destroy it except by some action not provided for in the instrument itself.
Hope this helps! :D
(I know I'm probably too late to be able to help you but I want to be able to help whoever comes across this question!)