He believed:
1) the Union could not stand divided. Either slavery would take over the entire country OR we would have to phase it out by restricting it to the South and letting it slowly die out.
2) He believed slavery was wrong. He often didn't come right out and say this during his election and the early years of his presidency because he knew that many of his supporters were so-called "Negrophobes" that just didn't want slavery in their territory. But in his heart, he did believe slavery was wrong, and he was more vocal about it later on.
3) Once the Civil War had started, he believed the Union HAD to be preserved. He didn't want a war, but now that a war had started his chief concern was bringing the South back to the Union.
4) Going back to my point #2, he believed adamantly in the Declaration of Independence. He believed that all men WERE created equal, including blacks. He just wasn't sure yet if the country was ready to be intermingled with blacks and whites (even though it was wrong, he knew many people wouldn't be happy working alongside blacks.)