"We know that some Southern men do free their slaves, go North and become tip-top abolitionists, while some Northern Men go South and become most cruel masters.
When Southern people tell us that they are no more responsible for the origin of slavery than we are, I acknowledge the fact. When it is said the institution exists, and it is very difficult to get rid of in any satisfactory way, I can understand and appreciate the saying. I surely will not blame them for not doing what I should not know what to do as to the existing institution. My first impulse would possibly be to free all slaves and send them to Liberia to their own native land. But a moment's reflection would convince me that this would not be best for them. If they were all landed there in a day they would all perish in the next ten days, and there is not surplus money enough to carry them there in many times ten days. What then? Free them all and keep them among us as underlings. Is it quite certain that this would alter their conditions? Free them and make them politically and socially our equals? My own feelings will not admit of this, and if mine would, we well know that those of the great mass of whites will not. We cannot make them our equals. A system of gradual emancipation might well be adopted, and I will not undertake to judge our Southern friends for tardiness in this matter."
Lincoln in speeches at Peoria, Illinois
The history of the last 150 years has shown that Lincoln was indisputably correct. It took over 100 years for "the great mass of whites" to reach a point where they were willing to make blacks "politically and socially (their) equals." Sad but true. In a democratic society it is not possible for any policy to move forward faster than it is accepted by "the great mass" of voters.
As is turned out, "a system of gradual emancipation" was adopted, though it didn't really hit its stride till the 1940s/1950s, when the unacceptable comparison between previously generally accepted "white supremacy" and Nazi racial doctrines became unavoidable.
Had Lincoln proclaimed racial beliefs that are widely accepted today among conservatives, he could never have been elected to any office during his lifetime, much less President.
His later comment in the same speech is also highly relevant. "Well I doubt not that the people of Nebraska are, and will continue to be as good as the average of people elsewhere. I do not say the contrary. What I do say is, that no man is good enough to govern another man, without that other's consent. I say this is the leading principle—the sheet anchor of American republicanism.
While Lincoln was not in favor of full political and legal equality for blacks, he was totally opposed to the institution of slavery and in particular its spread. Doubtless why Douglass respected and revered him.
"This is a world of compensations; and he who would be no slave, must consent to have no slave." - April 6, 1859 Letter to Henry Pierce.
" I am naturally anti-slavery. If slavery is not wrong, nothing is wrong. I can not remember when I did not so think, and feel. And yet I have never understood that the Presidency conferred upon me an unrestricted right to act officially upon this judgment and feeling." - April 4, 1864 - Letter to Albert Hodges
"The Autocrat of all the Russias will resign his crown, and proclaim his subjects free republicans sooner than will our American masters voluntarily give up their slaves." - August 15, 1855 - Letter to George Robertson
"Slavery is founded in the selfishness of man's nature -- opposition to it is in his love of justice. These principles are an eternal antagonism; and when brought into collision so fiercely, as slavery extension brings them, shocks, and throes, and convulsions must ceaselessly follow. Repeal the Missouri Compromise -- repeal all compromises -- repeal the declaration of independence -- repeal all past history, you still can not repeal human nature. It still will be the abundance of man's heart, that slavery extension is wrong; and out of the abundance of his heart, his mouth will continue to speak." - October 16, 1854 Speech at Peoria
"When Judge Douglas says that whoever, or whatever community, wants slaves, they have a right to have them, he is perfectly logical if there is nothing wrong in the institution; but if you admit that it is wrong, he cannot logically say that anybody has a right to do wrong." - October 13, 1858 Debate at Quincy, Illinois