I highly recommend the book The Fiery Trial about the development of Lincoln's beliefs on slavery. And he didn't take an "abolitionist" stance until later in his presidency. But he was always anti-slavery.
His famed "House Divided" speed was in 1858: LINK
"I believe this government cannot endure, permanently half slave and half free.
I do not expect the Union to be dissolved -- I do not expect the house to fall -- but I do expect it will cease to be divided.
It will become all one thing or all the other.
Either the opponents of slavery, will arrest the further spread of it, and place it where the public mind shall rest in the belief that it is in the course of ultimate extinction; or its advocates will push it forward, till it shall become alike lawful in all the States, old as well as new -- North as well as South."
Also in 1858, one of the primary issues of the Lincoln-Douglas Deabtes was slavery: LINK
Historian Stephen Oates noted that Mr. Lincoln "complained bitterly that race was not the issue between him and Douglas. The issue was whether slavery would ultimately triumph or ultimately perish in the United States. But Douglas understood the depth of anti-Negro feeling in Illinois, and he hoped to whip Lincoln by playing on white racial fears. And so he kept warning white crowds: Do you want Negroes to flood into Illinois, cover the prairies with black settlements, and eat, sleep, and marry with white people? If you do, then vote for Lincoln and the 'Black Republicans.'"
But whether the civil War is "about" slavery is of course overly reductive, but yes, it was the primary issue. Want to know how we know this? Because the Southern secessionists told us so. They made no secret that the reason for secession was to preserve slavery and white supremacy. It was the over-arching debate in American politics for nearly 40 years. No one was being coy.