I am not saying that all literacy tests are necessarily racial tests. All that I am saying is that Congress is empowered by Section 2 to enforce the Fifteenth Amendment by preventing a state from having a racial test for voting even though the state chooses to call it a literacy test. When, in March, 1965, 6.7% of adult blacks were registered to vote, 69.9% of adult whites were registered to vote.
How could you even begin to explain a disparity like that without considering the possibility that the state of Mississippi was violating the Fifteenth Amendment by using race as an important criteria in determining who was allowed to vote?
If, instead of Congress, the Constitution had empowered you to enforce the Fifteenth Amendment, what would you have done in response to those statistics?