I never heard that Catholics sympathized with the south. Never in all my born years if you’re talking about the Civil War era. Perhaps you’re mistaking the Copperheads of NYC for some Catholic cabal.
I happen to like and enjoy the south and it’s history but it most certainly is not a Catholic thing. Now we’re blaming Catholics for slavery? Blacks blame Jews, Protestants blame Catholics...sheesh, it never ends.
“I never heard that Catholics sympathized with the south. “
I suspect that the Catholics who lived in the South did. There doesn’t seem to be any record of Catholics in the South opposing the Confederacy.
Unionist activities in the South are pretty well known, including some like the Free State of Jones that weren’t all they are cracked up to be.