Saint Agatho played a crucial role in restoring unity between Rome and Constantinople and strongly opposed the Monothelist heresy, paving the way for its condemnation at the Sixth Ecumenical Council.
On June 27, AD678, Pope Agatho was consecrated as the seventy-ninth head of the Catholic Church and successor to Saint Peter. During Agatho's reign, the emperor withdrew support for the Monothelist heresy and restored relations between Rome and Constantinople. Shortly after his election, Agatho received a letter from Emperor Constantine IV proposing a council to address Monothelitism. The emperor recognized that Monothelitism failed to resolve the Monophysite heresy, which claimed Christ had only a divine nature.
Pope Agatho welcomed and approved Emperor Constantine's proposal but recommended holding preliminary synods in the Western Church to prepare for the conference. He emphasized the need to establish a firm stance against the Monothelist heresy before meeting with the Eastern Church in Constantinople. As part of this preparation, synods were convened in Rome in AD 680, followed by Milan, England, and Gaul, where delegates were chosen to attend the conference.
The delegation he sent to Constantinople as his representatives to attend the conference included two future popes, Pope John V and Pope Constantius. Pope Agatho and his delegation sent two extensive documents to Constantinople. The first was a letter from Agatho to Emperor Constantine IV, condemning Monothelitism as false and reaffirming the Holy See’s duty to uphold the true faith. The second was a decree, signed by 150 bishops at a synod in Rome, officially rejecting Monothelitism.
To address the challenges posed by Monothelitism, the emperor elevated the meeting to an ecumenical council in Constantinople, held in his palace. Presided over by Emperor Constantine IV, the council took place from November 7, 680, to September 16, 681, and became known as the Fourth Council of Constantinople.
Before the Fourth Council of Constantinople began, the emperor removed the Monothelist Patriarch of Constantinople and appointed a new patriarch who adhered to the canonical teachings of the Church. Additionally, the emperor instructed the new patriarch to ensure that all bishops under his jurisdiction attended the council.
At the thirteenth session of the council, Monothelitism was officially condemned as heresy, and its leaders, including Pope Honorius I, who had embraced the doctrine, were publicly denounced. Despite opposition from the papal legates to the condemnation of Pope Honorius I, the council proceeded without yielding. Additionally, the emperor withheld recognition of Pope Leo II's election, Agatho's successor, until the papal legates withdrew their objections to the council's decisions. The council affirmed the doctrine that Christ possessed two wills—divine and human—just as He had two natures—divine and human. In 683 AD, Pope Leo II confirmed the council’s teachings in a letter to Emperor Constantine IV. Pope Agatho passed away eight months before the council concluded, but the Council Fathers acknowledged in a letter to the emperor that Peter had spoken through Pope Agatho.
During his reign, Pope Agatho reached an agreement with the Exarch of Ravenna, ensuring that future metropolitans of Ravenna would be consecrated by the pope and receive the pallium. He also secured an exemption from the emperor’s tax on papal elections, allowing direct imperial approval of papal elections. Additionally, Pope Agatho supported the Bishop of York in his appeal against the division of the Diocese of York into three dioceses.
Pope Agatho was widely recognized for his benevolent and enthusiastic nature. He passed away on January 10, 681, during a plague outbreak in Rome and was laid to rest the same day in St. Peter's Basilica.