I guess you are confused.
We should discard those things that are against the constitution, and keep the things that are in the constitution.
Slavery is against the ideals that the Bill of Rights states - and is also against the laws that the Constitution decrees. The separation of Church and State is a "made-up" amendment to the Constitution that has taken root in liberal folklore - and has spread into Libertarian circles. Its time to put this fairytale to bed.....
Some founding fathers were slaveholders. The BoR was for white men. It was not unconstitutional until after a civil war and an amendment.