Up until Harriet Beecher Stowe wrote Uncle Tom's Cabin, abolitionism was very much a minority pursuit. Even those personally repulsed by slavery and everything that it stood for found abolitionism a tad extreme, the preserve of sour-faced Puritans and religious fanatics.
To a large extent, such attitudes were the product of ignorance. Those who were anti-slavery but not particularly enamored of abolitionists tended to hate slavery in the abstract. They assumed, quite rightly, that slavery was an evil and that life for those unfortunate enough to be slaves was sheer hell. But they never seriously acquainted themselves with slavery as it was experienced by actual slaves themselves.
That's why Uncle Tom's Cabin was so important. Though a work of fiction, it was based on real-life experiences of slavery. And so in reading the book, millions of Americans, for the very first time, became acquainted with the daily horrors endured by millions of people. Although it's a gross exaggeration to say that Uncle Tom's Cabin started the Civil War, it certainly stiffened the resolve of those opposed to any kind of political compromise with the South over the issue of slavery.
Prior to the publication of Uncle Tom's Cabin, it was possible for Americans to stay out of the slavery debate. But after the book was published and became a national and international best-seller, that became much harder. Many of those Americans who read the book felt that they now had to take a firm stand, on one side or the other. Inevitably, this deepened the already deep divisions in American society, which helped to create the conditions for the outbreak of the Civil War.