Who was Elizabeth I and why was she important for religion?
Elizabeth I was born on September 7, 1533 and she died on March 24, 1603. She is best known as the Queen of England and she played an enormous role in the area of religion. In short, she steered England permanently to break away from Rome (Catholicism) and she made England into a Protestant nation. Her legacy still continues to today and arguably her role in the rise of Protestantism in England was one of the most important actions in English history.
Here is a brief history. Henry VIII first broke away from the Catholic church on account of his marriage problems. When this took place, England left the Catholic fold, but there were still connections with Rome. When Edward VI took over at the age of 8 at the death of Henry VIII, the nation still remained Protestant due to Edward's Protestant advisers. However, the situation was still far from certain. When Mary became queen at Edward's death, the nation turned more Catholic and there was even a persecution of Protestants.
This is where Elizabeth comes into the scene. She was able to steer England away from internal conflict over religion, while creating a Protestant nation. This would be significant in the coming years and England would become the most prosperous Protestant nation.
I will also add a link to a good biography of Queen Elizabeth.
The above answer fails to mention that Elizabeth I was the daughter of Henry VIII by Anne Bolyen. It was the fact that she was a girl (and that her brother was still-born) which led Henry to rid himself of Anne Boleyn by having her executed. She was named:
supreme governor of this realm in all things ecclesiastical and temporal
by the Act of Supremacy of 1559; however she would not accept the title of "Supreme Head of the Church." She stated that that dignity belonged to God alone. She established a Protestant church; but retained sufficient elements of Catholic practice so that Catholics could worship there without being greatly offended. In a major concession to Catholics, she retained Matthew Parker as Archbishop of Canterbury, although he was Catholic and had been appointed by her Catholic half-sister Mary 1. She also retained two of the five sacraments recognized by the Catholic Church.
Elizabeth's actions displeased Pope Pius V who excommunicated her. The order of excommunication absolved Elizabeth's subjects from obeying her; however they were so dedicated to her that they ignored the excommunication.