The Investiture Controversy marks one of the most unstable moments in the history of the Catholic Church. At the heart of the controversy, there were two sides competing for supreme authority, trying to answer the question of whether the king's power outranks that of the Pope's or vice versa. On the one side, there was King Henry IV, son of the late Holy Roman Emperor Henry III. On the other, there was Pope Gregory VII, formally Hildebrand of Sovana, who led a strong reform campaign in the Catholic Church. The controversy first arose after Henry directly disregarded the warning of the Pope to not associate with those who violated the Papal Decrees of 1074-75.
[...] both the political authority and spiritual authority to pass his decrees of reform. Secondly, according to many of the doctrines of the Christian faith, holiness could not be bought. And thirdly, in defense of the Pope's excommunication of the king and others, the king was provided ample warning and numerous chances to repent and return to grace. In his argument that Gregory had assumed the Papacy unlawfully, Henry interprets the decree regarding Papal Elections of 1059 to mean that Papal elections must be overseen and ultimately approved by the king (48). [...]
[...] As time progressed, however, the Pope began to notice that the king's words of allegiance were not exactly matching his actions as the king was seen to be associating with Archbishop Siegfried of Mainz, an excommunicated simoniac. Growing weary of the king's mixed signals, the Pope chose to offer him an ultimatum asking him to cease his association with the excommunicated and seek penance or else he, himself, would be excommunicated. Evidence of this can be found in Gregory's letter to Henry when he states, If this is true, you yourself know that you cannot receive the favor of God nor the apostolic blessing unless you shall first put away those excommunicated persons and force them to do penance and shall yourself obtain absolution and forgiveness for your sin by due repentance and satisfaction. [...]
[...] After a new Pope was selected he would then have the power to promote the king to Holy Roman Emperor. Under these actual regulations, Pope Gregory VII did assume the Papacy quite lawfully and democratically, as was intended by the previous leaders of the Church. The second part of Gregory's adversaries' argument was that the Pope did not have the spiritual authority and that it was unlawful to excommunicate others. These claims were also incorrect because as an heir to the powers granted to St. [...]
using our reader.