Maybe we should start at the beginning.
Benedetto Gaetani was born about 1235 in Italy. A younger son of minor nobility, his religious career began when he was sent to a monastery. He became secretary to a cardinal in 1264, which put him close to Vatican politics. He had a busy career in international affairs, accompanying a cardinal to England to put down a rebellion, going to France to supervise a collection, and acting as a diplomat to France, Naples, Sicily and Aragon.
In December of 1294, stating "the desire for humility, for a purer life, for a stainless conscience, the deficiencies of his own physical strength, his ignorance, the perverseness of the people, his longing for the tranquility of his former life," Pope Celestine V resigned. A contemporary said that it was Benedetto Gaetani who convinced him to resign; other reports say that he was only one of several, or that Gaetani was the person who convinced Celestine that a papal resignation was legal. Whatever the case, Benedetto Gaetani was elected pope by the conclave on Christmas Eve.
That's when the fun began. His first act was to imprison his predecessor. A few years later he formalized the Roman Jubilee, a tradition that established a year of pilgrimages to Rome for pardoning of sins. This influx of tourists seeking forgiveness turned into a big money-maker for the pontiff. This may be why Dante put Boniface into the 8th circle of Hell in Inferno, with the simonists. He enriched the lives of his relatives, and used his position to war against the Colonna family (rivals of the Gaetani); he even offered a pardon for one's sins equivalent to that granted when one goes on Crusade if you would join in his war against the Colonna. He was the epitome of a power-mad ruler.
Then, on 18 November 1302, Boniface made his boldest move: the papal bull called Unam Sanctam (One Faith). The document establishes that salvation is only available through the Church, and that the Church wields "two swords" that represent both spiritual and temporal power. Among other pronouncements, it concludes with
Furthermore, we declare, we proclaim, we define that it is absolutely necessary for salvation that every human creature be subject to the Roman Pontiff....and that has been the source of endless conflict between the papacy and others ever since.