What a politician, like Lincoln, "really thinks/believes" is often a conclusion of his opponents and his adherents with their own agendas long after the demise of the politician.

I prefer contemporaneous documentation of the time over vague assertions about what modern-day people of any persuasion want to be the truth.

The Pentagon Papers, the Nixon tapes, the Snowden WikiLeaks of our more modern times clearly demonstrate that lies, obfuscation and deception are the tools of politicians of every political persuasion.  

I read an interesting book about Lincoln where it was pointed out that he co-opted and de-fanged several of his Presidential opponents by putting them in his cabinet and the Supreme Court.  He was a consummate politician.

The Lincoln debate quote I gave was made one year before Oregon was admitted to the Union with its prohibition against both slavery and the presence of black people.   The times and the politics were grossly different.