Why does Dr Martin Luther King Jr's tombstone say Free at last free at last Thank God almighty I am free at last?

I think the phrase was written on his tombstone because the Words were part of his famous, "I Have a Dream," speech. He closed with the Words of an old "Negro" spiritual, "Free at last, Free at last, thank God Almighty, I am free at last."

This quote from King's most powerful speech was also symbolic of death representing freedom from earthly concerns.