Lincoln did not oppose slavery completely and absolutely. He wrote a letter stating that his goal was to preserve the Union, and that if he can preserve the Union by preserving slavery, he would do that, but if he can preserve the Union by abolishing slavery, he would do that. When he wrote the Emancipation Proclamation, he did so as a war-time measure, and admonished abolitionist zealots against preying on the slave-owners of Kentucky, Maryland, et al., because they were not in rebellion, so did not fall within the purview of the Proclamation. He also offered the secessionist states, prior to Sumter, to guarantee that they may remain slave states forever, so long as no new territory would be allowed to become a slave state. They rejected the offer. But he made it.
Lincoln opposed slavery on moral grounds. Opposition cannot be any deeper or absolute than this.
We can review his commentary if needed.