You write 'was' when asking about the power of the Roman Catholic church, the oldest institution in the western world. It is true that this power has been considerably lessened since the revelations about sexual abuse by priests, nuns and monks. Prior to this, the concept of someone who was above all earthly things such as sexual desire, greed, ambition and so on was very compelling.
Riches poured into the church from believers and riches were displayed in the church to dazzle and impress. Catholics believe that the pope acts as direct descendant of St Peter who Christ made the first appointed head of the church. Millions believe(d) in the absolute purity as well as authority of the pope.
Today, the Catholic Church is the biggest in Christianity with over a billion members around the world. It is one of the oldest religions. When Christianity evolved from Jesus Christ’s death in 1 AD, Christianity spread in the Roman Empire. Eventually, throughout time, Catholicism was the first branch of Christianity. Later, during the Protestant Reformation, other denominations would be founded.
However, only citizens of Rome and then other areas like England forced its citizens to only be able to worship the Roman Catholic Church. During this time in the 7th centuries and beyond, England and other countries had governments that were strongly tied to the Catholic Church. The priest, pope and other religious leaders were just as powerful as the government leaders.