The bull belongs to a dispute about money and jurisdiction rather than about theology. Philip IV of France had been taxing the clergy of his kingdom to pay for war with England, and had then arrested and tried a French bishop, Bernard Saisset, on a charge of treason. Both acts touched claims the papacy regarded as its own: the immunity of church property from lay taxation, and the right of the church to try its own clergy. Boniface had already answered the taxation in 1296 with Clericis Laicos and had been forced to soften it. Unam Sanctam, issued on 18 November 1302, restated the underlying principle at its widest.
Philip's answer was not argument. His counsellors accused the pope of heresy and simony and called for a council to judge him. In September 1303 a force under Guillaume de Nogaret and members of the Colonna family seized Boniface at Anagni. He was released within days but died on 11 October, and within a decade the papal court had moved to Avignon. In 1306 Clement V issued the brief Meruit, which stated that France stood in no worse position with respect to the church than before the bull, withdrawing its political edge without touching a word of its text.
That sequence is why the document is read two ways. As a statement about communion with Rome it was never withdrawn and is still quoted. As a claim to direct authority over kings it was defeated within a year of being made, and the medieval papacy never recovered the ground it had staked.