King, in general, loved to use allusion throughout his work, and this speech is no different: at multiple points he makes reference to various sources in support of his argument. King was part of a community of racial justice leaders and orators and drew important parts of his speech from them. These included Archibald Carey’s address at the 1952 Republican National Convention, which also included “My Country ’tis of Thee.” King’s long story of his “dream” was inspired by Prathia Hall’s speech in 1962 at a church in Georgia burned down by the Klan, in which she repeated the phrase “I have a dream.” These allusions emphasize King's respect for his predecessors in the civil rights movement.
King's speech also made allusion to more historical sources, including Abraham Lincoln and William Shakespeare. The following lines are a reference to the famous opening of Richard III, Shakespeare's play of the murderous king of England during the Wars of the Roses: "The sweltering summer of the Negro's legitimate discontent will not pass until there is an invigorating autumn of freedom and equality." Shakespeare begins his play similarly: "Now is the winter of our discontent / made glorious summer by this sun of York." King makes use of an elegant and familiar phrase in Shakespeare to make an appealing description of "the Negro's legitimate discontent." The allusion also expands an extended metaphor in the speech, of heat as a symbol of the persecution of Black Americans.
As in his reference to Lincoln at the beginning of the speech, King's allusions overall serve to assert his connections to the movement of racial justice and his position of authority within it.
King, a Baptist minister, frequently referenced the Bible to great effect. In this speech, King uses direct allusions to the Bible (in addition to general Christian imagery) to elevate the imagery and argument of his speech, from a question of racial justice in America to one of world peace. One of the most famous lines of the speech is a direct quotation of Isaiah 40:4–5:
I have a dream today that every valley shall be exalted, every hill and every mountain shall be made low, the rough places shall be made plain, and the crooked places shall be made straight and the glory of the Lord will be revealed and all flesh shall see it together.
The Book of Isaiah, in the Old Testament, describes God's plan for peace on Earth, should humans only put it into action. King quotes Isaiah not only because it would be familiar to many of his listeners, but also for its succinct and evocative description of his message: in the world of racial equality that he dreams of, there will be a more general peace on earth. King's dream is not only for racial justice, then, but for world peace.
King also uses references from the Bible to emphasize the importance of his message. In the last paragraph of his speech, King makes his call to action, saying that only through the collaboration of every American is justice possible:
And when we allow freedom to ring [...] we will be able to speed up that day when all of God’s children—black men and white men, Jews and Gentiles, Catholics and Protestants—will be able to join hands and to sing in the words of the old Negro spiritual: "Free at last, free at last; thank God Almighty, we are free at last."
This alludes to Galatians 3:28: “There is neither Jew nor Greek, there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus.” By evoking this line, King implies that his vision of an America with true equality is as sacred as the very idea of Christian brotherhood and unity. King's biblical references, here and throughout the speech, show his belief that his dream of equality has both political and religious importance. The references to the Bible also turn his argument for racial justice into a larger one, in support of a better future for the world and all people.