Dr. Martin Luther King Jr. Speech(I Have A Dream)

By the content of their character...

He is quoting words spoken by the prophet Isaiah, Chapter 40:4-5 at the end of the speech.

Isa 40:4-5 Every valley shall be exalted And every mountain and hill brought low; The crooked places shall be made straight And the rough places smooth; The glory of the LORD shall be revealed, And all flesh shall see [it] together; For the mouth of the LORD has spoken." (NKJV)