Martin Luther King wrote this letter on April 16, 1963. One year later, Congress passed the Civil Rights Act of 1964. These acts prohibited any institution involved in interstate commerce to withhold services based on race. The clergymen who criticized King for his nonviolent movement would most likely have never been the beneficiaries of these acts had King decided to listen to them. Acts like these prove the effectiveness of King's movement.
This once again shows how King wanted to change practices that he felt were unjust. While the clergymen were content to follow laws that they knew to be unfair, King was not. He would not be deterred from his course until he saw justice.
Congress passed the Civil Rights Act of 1964 using the power given to them under the Interstate Commerce Clause and the Elastic Clause in the Constitution. The Interstate Commerce Clause allowed Congress to regulate anything having to do with interstate commerce (a definition Congress took full advantage of) and the Elastic Clause allowed Congress to pass any laws that it thought were necessary and proper to fulfill its constitutional duty.
GLOBAL CONTEXT
It is important to understand the global context in which this act was created. The 1960s was a time of not only civil reform, but of the Cold War. In order for the United States to maintain its image of democracy to the rest of the world, it was necessary that its laws should be in accord with its platitude of democracy. In the article "Desegregation as a Cold War Imperative", which appeared in the Stanford Law Journal, it is shown that these acts were passed by Congress not only to confer to the Negro rights that should have been his in the first place, but also to preserve the image of the United States.
Link to
Explanation of Brown v Board of Education
Link to Civil Rights Act of 1964