The Council of Thessalonica was a council of the Christian Church convened in 635 in response to the Third Council of Constantinople held the previous year. It rejected the decisions of the latter council as heretical, excommunicated all those churchmen and secular officials who had supported those decisions, and released all citizens of the Roman Empire from any oaths of loyalty they may have given to Emperor Heraclius or his allies.
When the decision of the Third Council of Constantinople to accept Islam became known, there was shock and outrage throughout the empire. Devout churchmen condemned Heraclius fiercely, as well as those whom they saw as apostates and traitors to Christianity, and in many cases they were supported by secular local government officials and by the general populace. In January of 635 the Army of Moesia rose up in open revolt by proclaiming their general, Heraclius' brother Theodorus, as Emperor, and they were soon joined by most of the other army units in Europe.
Heraclius was forced to flee Constantinople amidst rioting and insurrection, abandoning the city to Theodorus. After the Ecumenical Patriatch was found murdered two weeks later, Theodorus invited Macedonius, the zealous former Patriarch of Antioch, to take his place. Macedonius immediately summoned all those bishops who had remained loyal to Chalcedonian Christianity to a new council, where he hoped to undo the damage to the church.
Those bishops who had accepted Islam were not invited, and the council was mostly ignored by the Germanic clergy of the west, who viewed it at the time as a minor Roman internal matter. As a result, those attending were nearly all staunch Chalcedonian conservatives from Greece, Italy and Africa, and Macedonius had no problem bending them to his will.
The council was later condemned after Heraclius' victory, and it is not formally recognised as ecumenical by the modern Catholic Church. Nevertheless, it remains notable as one of the key early events that led to the decisive split between Christianity and Islam.