Ever hear the saying "Absolute power corrupts absolutely"? It means that when people's authority goes unchecked, all that power can go to their heads and lead them to do some pretty nasty things. This is more or less the situation in The Golden Compass: the Church has power over everything, in the form of a body of warring factions and agencies known as the Magisterium. The most powerful unit is the Consistorial Court of Discipline, or the General Oblation Board (the Gobblers). As we will learn, this particular arm of the Church, headed by Mrs. Coulter, is notable for its evil acts.
Power, used wisely and in moderation, isn't necessarily bad, though. We see some good rulers in the form of Iorek, the king of the bears, and Serafina Pekkala, the queen of the witches.
Though Lyra may seem to be a powerless child, her clever mind and silver tongue give her a great deal of power, which she uses to her advantage over many characters, including Iofur and Mrs. Coulter.
Through encountering cruelly powerful characters like Mrs. Coulter, Lyra develops from being a bully to being a protector.