I have found this phrase on New York Times recently. And I couldn't figure out the meaning of let freedom ring. I tried searching the meaning on the dictionary too.
Though I may not be here with you, I urge you to answer the highest calling of your heart and stand up for what you truly believe. In my life I have done all I can to demonstrate that the way of peace, the way of love and nonviolence is the more excellent way. Now it is your turn to let freedom ring.
Best Answer
This is the final line from the first verse of the patriotic song America (My Country, 'Tis of Thee). This song is extremely well known in the US and is sung nearly as often as the actual national anthem, so in part, the author is using this phrase to evoke a feeling of patriotism and love of country simply by referencing a patriotic song that all Americans would know.
The phrase is using the meaning of the verb to ring: "to make or cause to make a clear vibrating sound" metaphorically. Freedom itself is not a sound, but "letting freedom ring" means to exercise your freedom clearly and openly, in this case by standing up for a just cause in a nonviolent way.