The previous thoughts were well conceived. I would say that that the abolitionist movement ended up galvanizing and demarcating strict boundaries or sides regarding the nation's stance on slavery. Indeed, it was not as if all the individuals in the North believed in equality. Rather, the abolitionist movement that was located in the North ended up passionately speaking about the issue of slavery as something more than tradition or Status Quo. Abolitionists were out there and defiantly making the case that slavery was a moral evil. The Grimke Sisters, Frederick Douglass, William Lloyd Garrison, John Brown and others were completely committed to the idea of abolishing slavery and demonstrating to as many people as possible that it was morally and politically unacceptable for a nation with precepts as America to permit slavery. To this end, Southerners took grave exception with such a depiction and became further entrenched in their practice of slavery as a result of their own pursuit of freedom and happiness. The abolitionists thus found more of an audience in the North, contributing to why the region became more of a seat for abolitionist thought.
It's important to note that most northerners did not believe in black equality, even after the Civil War. In general, this was a racist country, and would remain so for quite some time. The abolitionist movement was a minority, even by 1861 when the war started. Most northerners believed that slavery was OK if a state wanted it, they just didn't think it should spread farther west into new territories.
What the abolitionist movement did, and this took a considerable amount of time to achieve, was that it raised awareness of slavery, and how brutal the institution was. The movement did grow, especially in the 1850s, due to influential speakers, writers and the influences of the churches. But many abolitionists, like John Brown or William Lloyd Garrison, were dismissed by the mainstream of the North as too radical, and their influence was therefore more limited than that of say, Frederick Douglass or Harriet Beecher Stowe.