In February 1570, Pope Pius V declared that Elizabeth was a heretic and, as such, she was excommunicated by way of a Papal Bull (order). The Bull released Catholics from any loyalty to Elizabeth and called upon them to remove her from the throne.
The Pope was trying to capitalise on the discontent caused by the arrival of Mary, Queen of Scots in England, as well as the recent rebellion of the Northern Earls. This was a danger to the Elizabethan Religious Settlement and could provide a reason for a Catholic uprising, possibly supported by a foreign power.
However, the Pope had miscalculated. English and Welsh Catholics preferred to keep quiet about their religious beliefs and remained loyal to Elizabeth. The Pope had seriously overestimated his power over Catholics in these countries. They wanted to continue to live and worship as Catholics but most did not want the Pope to have political power.
Despite this, Parliament took no chances and in 1571 passed a series of Acts designed to protect Elizabeth from any consequences of the Papal Bull.