The Pope is probably also against slavery…
Lincoln was ambivalent to slavery, at least as POTUS. The Emancipation Proclamation was more of a military gambit than anything.
Lincoln was unambiguously opposed to slavery; he just had a number of higher priorities. He was willing to tolerate slavery if that’s what it took to keep the Union together, but in such a case, he viewed it as the lesser of two evils. And in any event, it turned out that his priorities aligned, and that the best way to preserve the Union was to stamp out slavery anyway.