The Third Council of Constantinople was an ecumenical council of the Christian Church that met in Constantinople in 634. It provoked a schism between the eastern and western churches and is often considered to mark the beginning of the official separation between Islam and Christianity.
In 632 the Emperor Heraclius travelled incognito to Mecca, where he spoke with the Prophet Muhammad, listened to his last sermon, and promptly converted to Islam. While in Arabia he arranged for missionaries to evangelize the people of the southern empire, but he still wanted to do something to make his conversion official.
At this time Heraclius, along with most of the population of the Middle East, considered Islam to be an extreme non-trinitarian form of Christianity, similar to the Monophysite form practised by the majority in Syria and Egypt. Upon his return to Constantinople, therefore, he invited all the bishops of the Church to attend an ecumenical council where the orthodoxy or heterodoxy of the creed of Islam would be discussed. By going through the traditional channels for ecclesiastical reform, he hoped to spread his faith to the greatest possible number of people with the least possible amount of resistance.
The Council began on 7 August 634, attended by representatives from all over Christendom. Heraclius spoke first, followed by the Patriarchs of Constantinople and Alexandria, who both supported his position. A representative from Caliph Ali, a man named Abu Bakr who had long been a faithful companion and the father-in-law of Muhammad, was then brought forth and questioned about his beliefs.
Heraclius and Abu Bakr were supported by most of the bishops of Syria and Egypt and were strongly opposed by those of Italy, Greece, Africa and Asia Minor. Few were able to make the long journey from Gaul, Hispania or Britain, but it's understood that the Franks were also opposed to the motion. However, late in the council Heraclius received the unexpected support of the Arian Lombards and the Church of the East, which allowed him to gain a majority and have the Shahadah declared the one, holy, catholic and apostolic creed of the Church.
As soon as the decision of the Council became known, Greece and Anatolia erupted in rebellion, led by Heraclius' own brother Theodorus. Heraclius eventually defeated the uprising, enabling him to begin putting his new religious policies into place, but then the Exarchates of Italy and Africa rebelled as well, proclaiming the restoration of the Western Roman Empire.