How is "Nicholas Ridley" an allusion in Bradbury's Fahrenheit 451?
An allusion is a reference to a well-known person, place, work of art, idea, etc., that the audience is hopefully able to relate to through foreknowledge, to find similarities when used in a comparison. (If the allusions is too vague, it is not effective.)
Before Henry VIII of England died, he had severed all ties with the Catholic Church in Rome and created his own religion—a Protestant faith, called the Anglican church—partly because he wanted a divorce. Eventually Henry's daughter, Mary, by his first wife Catherine of Aragon (who he had divorced, and who was a staunch Roman Catholic) came to the throne. Like her mother, she was also a supporter of the Catholic Church, but she was rabid in her resolve to make England Catholic once more. In fact, she was known as "Bloody Mary" because of all the Protestants she had put to death. Ridley (an Anglican Bishop) was a part of the religious turmoil that followed Henry's death because he refused to give up his faith. He and Bishop Hugh Latimer were both burned at the stake. Latimer is believed to have said to Ridley before he died:
Be of good comfort, Master Ridley, and play the man; we shall this day by God’s grace light such a candle in England, as I trust shall never be put out.
Fire is a prevalent image in Ridley's last days—even in this quotation. Ridley defied the law of the land—Queen Mary—by rebelling against her religious dictates. Fire is also prevalent in Montag's life; he, too, defies the government's laws. Both men are prepared to sacrifice everything for what they believe. However, where Ridley dies, Montag escapes. Where fire took Ridley's life, fire ultimately saves Montag's life as he kills Beatty who is ready to arrest him, and the Mechanical Hound who is pursuing him.
As Latimer noted, Ridley would be one of many to stand up for what he believed so that his faith would live on after his death. Montag stands up for what he believes, and he ends up joining others who plan to rebuild their society that has been destroyed by government-sanction bombings. He and others will carry fire—a torch—to make certain that the knowledge contained in books does not disappear. Granger notes:
...that's the wonderful thing about man; he never gets so discouraged or disgusted that he gives up doing it all over again, because he knows very well it is important and worth the doing.
Alluding that Montag is similar to Ridley is very effective in Fahrenheit 451.