Complex sentence: Before the war, many religious leaders in the slave states, such as South Carolinian James Henley Thornwell, supported slavery, denouncing those opposed to slavery, such as the Republicans, as being against Christianity for doing so.
Split into simpler sentences: Before the war, many religious leaders in the slave states, such as South Carolinian James Henley Thornwell, supported slavery. These clergymen often denounced those opposed to slavery, such as the Republican Party, as being against Christianity for doing so.