What did Abraham Lincoln mean by A house divided against itself cannot stand"?"

On June 16, 1858, the Republican State Convention met in Springfield, Illinois, and chose Abraham Lincoln to run against Democrat Stephen Douglas for the U.S. Senate. Lincoln's speech that evening was specifically about the problems of slavery in the United States, and especially the effect of the recent Dred Scott decision of the U.S. Supreme Court.

Lincoln paraphrased the following passage from the Bible, Matthew 12:25, when he spoke of a house divided:

And Jesus knew their [the Pharisees'] thoughts, and said unto them, "Every kingdom divided against itself is brought to desolation; and every city or house divided against itself shall not stand."

Lincoln hoped to use a well-known figure of speech to help rouse the people to recognition of the magnitude of the ongoing debates over the legality of slavery. His use of this paraphrased metaphor is perhaps clearer when you look at some more of his speech:

"A house divided against itself cannot stand." I believe the government cannot endure permanently half slave and half free. I do not expect the Union to be dissolved — I do not expect the house to fall — but I do expect it will cease to be divided. It will become all one thing or all the other. Either the opponents of slavery will arrest the further spread of it, and place it where the public mind shall rest in the belief that it is in the course of ultimate extinction; or its advocates will push it forward till it shall become alike lawful in all the States, old as well as new — North as well as South.

As you can see, in this metaphor, the "house" refers to the Union — to the United States of America — and that house was divided between the opponents and advocates of slavery. Lincoln felt that the ideals of freedom for all and the institution of slavery could not coexist — morally, socially, or legally — under one nation. Slavery must ultimately be universally accepted or universally denied.

This speech, given two-and-a-half years before South Carolina would become the first state to secede from the Union, foreshadowed the coming storm of the Civil War. Although Lincoln lost the election to Stephen Douglas, his eloquent political arguments put him in the national limelight and paved the way for his election to the presidency in 1860.