That's definitely a matter of debate and a complicated argument. But personally the evidence leads me to believe that he was an abolitionist, but publicly he spoke as a moderate in order to get elected.
The following passage from his letter to Joshua Speed as evidence:
You may remember, as I well do, that from Louisville to the mouth of the Ohio, there were, on board, ten or a dozen slaves, shackled together with irons. That sight was a continued torment to me; and I see something like it every time I touch the Ohio, or any other slave-border. It is hardly fair for you to assume, that I have no interest in a thing which has, and continually exercises, the power of making me miserable. You ought rather to appreciate how much the great body of the Northern people do crucify their feelings, in order to maintain their loyalty to the Constitution and the Union.
http://www.abrahamlincolnonline.org/lincoln/speeches/speed.htm
He was an abolitionist, he wanted slavery outlawed in the USA. He was however possibly willing to let it go on in order to avoid a catastrophic war. That doesn't mean he wasn't abolitionist by the actual definition of the word.
I think this is a good summary. If the South had stayed in the Union voluntarily, I think that it might have taken many years to abolish slavery in the South (by a slow, legal, democratic means culminating in a Constitutional amendment), but Lincoln, together with other abolitionist strong trends in the North, would have prevented expansion of slavery to the new territories and would probably have soon eliminated fugitive slave laws that required Northerner's to return escaped slaves to the South. But even this modest move toward progress, which would have taken even longer than did the all-too gradual anti-slave enlightenment of the North, was considered far too radical for the South. So they rebelled, initiated a war with the North, and lost. Probably further dooming it to a primarily rural economy.
So the South was its own worse enemy in this regard. But to suggest that the South was close to abolishing slavery on its own, or that the North was almost as pro-slavery as the South, is simply re-writing history.
I would also note that of the over 2 million Union solders, 92% were volunteers. To suggest that all these people signed up, fought and died but didn't see much of an ideological difference with the South, is just illogical.
I am not suggesting that the Northern did not include some people who were pro-slavery, or even a majority of racists (at the time it was very popular). But clearly the North and the South as wholes held very different views as to the legitimacy of slavery. Which the South was proud to verbalize during their declarations of secession. And which continued even after the Civil War as the North slowly became less willing to allow Jim Crow laws, and the South sought to solidify these, until the Federal government in the 1950s and 1960s intervened in the South.
Finally, as to flags. Yes, the USA flag was used during a history that included some good events and some very bad ones. Yet it was created to symbolize formation of a new nation, not specifically to symbolize the bad. It currently symbolizes the nation, for all of its good and bad., and no one created the American flag to expressly symbolize the bad, such as the destruction of the Native Americans. I don't know of any national flag that doesn't encompass some bad history as some good history. In contrast, the Confederate flag was created to symbolize a pro-slavery revolt, and later revived to symbolize segregation forever. It doesn't symbolize a country, and when asked as to the positive things that it symbolizes, its supporters have yet to come up with anything significant (typically a "Well, Southern Tradition, or States Rights." Yet when asked exactly what tradition or rights these were, the supporters fall strangely silent.
