The amendments are federal restrictions on federal power. The point of the 10th was to make that brutally clear. Funny how so many are confused by the bill of rights.What do you think that means?
It means that what the constitution prohibited of the states before the 10th Amendment (like requiring religious tests) or after it (like the prohibition upon unequal treatment before the law in the 14th Amendment) the states have no right to do.
Not it doesn't, the phrase is "prohibited by it to the States." That is clearly referring to things like slavery, which the Constitution specifically made a state issue. The Civil War effectively invalidated that small portion of the 10th Amendment and expanded the power of the federal government, but the restrictions still apply only the the feds.