During the time of the Islamic Prophet, Muhammad(Pbuh). A messenger was send to the Byzantine Emperor inviting him and the rest of the empire to join the Religion of Islam claiming it to be the final religion from God. They have killed the messenger. As back then and even today the custom amongst all nations was that you do not kill messengers for that is an open indication of war.
War started between the early Muslims from Arabia and the Byzantines but no conflict occurred. There was a time when the Prophet Muhammad moved his forces up north but they did not meet the Byzantines.
It is only After the Prophets death that a new Caliph (Successor to the prophet in leading the people ) was elected by the companions of the prophet and some helping tribes, the man becoming the Caliph was the best friend of The Prophet Known as Abu-Bakr. After successfully reconquering Arabia within 2 years Abu-Bakr had passed away and a new Caliph Umar-Ibn-Al Khattab came to authority. He was said to be a very powerful leader. They did not forget the time when the messenger was killed. But now with all of Arabia under Islam they were powerful enough to fight their enemies. Both the Byzantines and the sassanid Persians.
Conflict kept going until the rise of the Ottoman Empire who had finished what started years ago by taking over Constantinople. In Europe Islam was seen as a threat to Christianity so it was obvious to both sides that peace can not be attained and well as every Empire wanted to grow in land the Ottomans controlling most of the Muslim lands knew the best way to grow is to the west where their enemies are. And that is how Islam got in to conflict with Europe.
Okay i'm sorry for all my typos and spelling mistakes if there are any(Which probably are) And also this was a summary of what i have heard this mite be wrong information but it also mite be right i highly advice you to keep searching for answers and verify my information.
Islam did NOT spread from the Byzantine Empire; the regime was an Orthodox Christian theocracy. Some of the earliest expansions of the Rashidun Caliphate were at the direct expense of the Byzantine Empire.
The Byzantine Empire declined after Islam spread. The words "Islam spread" are nebulous. If the question is referring to the Rise of Islam period, when Mohammed and the Rightly-Guided Caliphs were spreading Islam, then yes, the Byzantine Empire decline after these events. If, however, you are referring to the Seljuk and Ottoman conquests of Anatolia nearly 600 years later which eventually ended the Byzantine Empire, the Empire was in decline long before these armies trampled the Byzantines to the ground. The answer is more complex than the above answers would purport. See the link below for another relevant answer.
The underlying premise of the question is incorrect. The Byzantine Empire never converted to Islam and Islam was, by and large, illegal within the Byzantine Empire for as long as it existed. The reason why the territory formerly controlled by the Byzantine Empire became Muslim-majority was because it was conquered by various Islamic States such as the Rashidun Caliphate, the Umayyad Caliphate, the Seljuq Turkish Empire, and the Ottoman Empire (which finished the job).
Islam
Islam became a rival of Christianity in the Byzantine Empire.
The Seljuks or the Sufi's
The Byzantine Empire declined after Islam spread. The words "Islam spread" are nebulous. If the question is referring to the Rise of Islam period, when Mohammed and the Rightly-Guided Caliphs were spreading Islam, then yes, the Byzantine Empire decline after these events. If, however, you are referring to the Seljuk and Ottoman conquests of Anatolia nearly 600 years later which eventually ended the Byzantine Empire, the Empire was in decline long before these armies trampled the Byzantines to the ground. The answer is more complex than the above answers would purport. See the link below for another relevant answer.
Islam
The underlying premise of the question is incorrect. The Byzantine Empire never converted to Islam and Islam was, by and large, illegal within the Byzantine Empire for as long as it existed. The reason why the territory formerly controlled by the Byzantine Empire became Muslim-majority was because it was conquered by various Islamic States such as the Rashidun Caliphate, the Umayyad Caliphate, the Seljuq Turkish Empire, and the Ottoman Empire (which finished the job).
400 - 1400, during the era when the Roman Empire declined, the Byzantine Empire flourished, and Islam was founded.
Islam
Islam became a rival of Christianity in the Byzantine Empire.
what two empires continued to spread Islam after the Arab Empire collapsed?
Islam became the strongest rival of Christianity in the Byzantine Empire.
Invasions by the Islamic Ottoman Empire destroyed the Byzantine Empire.
The Byzantine Empire, as well as the Persian Empire were located near the Arabian Peninsula, in the budding years of Islam.
The Byzantine Empire and the Sassanian Empire were so weak that the Islamic Rashidun Caliphate conquered 2/3 of the land of the former and completely conquered the latter.
Invasions by the Islamic Ottoman Empire destroyed the Byzantine Empire.