Possibly the greatest single calamity ever to befall the Christian Church was self-inflicted. Early in the 7th Century after Christ, an Arab named Mohammed either started a new religion or adapted one version of Christianity (Monarchianism) to a unique Arabian form and established what is generally called Islam, today.
“Islam” means “submission”, and until late in the 20th Century, the religion was generally known as “Mohammedanism”, but with our modern penchant for being offended, that name became offensive to some, and we all began to use the term “Islam”.
In any case, in the 630s, Arab armies burst out of the Arabian Peninsula and began a relentless march, conquering much of both the eastern and western Roman empires. In the west, they took all of North Africa, Sicily and most of Spain. In the east, they took most of what was then called Palestine, Persia and Syria.
This included, of course, Jerusalem, the home and historical center of the Church, and also included two of the original actual centers of the Church, Alexandria and Antioch. So, by 700 A.D., Mohammedan Arabs controlled three-fifths of ancient Christian lands.
That left only two of the historic Patriarchates, Rome and Constantinople. Constantinople faced Islam across a slowly shrinking border for more than 700 years. Several times, they tried to take back Jerusalem, but without success.
In 1095, the Patriarch of Rome, Pope Urban II was faced with a number of problems in “his” western Roman empire. Mainly, he found himself supposedly in charge of a number of violent Germanic kingdoms and principalities that made war upon one another as both their occupation and their hobby.
It was the age of “Knighthood”, and the only way a young nobleman could prove himself was by fighting and beating another knight, baron, count, or whatever. There were hundreds of these little warlords scattered all over Europe. At the same time, he was getting a lot of flak from Christians returning from Jerusalem, complaining that the Arabs were interfering with their visits to the Holy Land.
Urban had an inspiration: Why not send all those troublemaking knights off to Jerusalem, to fight Arabs and re-take the Holy Land. With a number of fits and starts, it worked, and eventually, two Christian kingdoms were established in Palestine. In the process, though, virtually all the western armies had to pass through Constantinople, and they ended up doing more damage there than an invading army might have.
Indeed, the Fourth Crusade, in 1202, originally designed to re-capture Jerusalem (which had been re-captured by the Arabs by this time) by invading through Egypt, ended up invading and sacking Constantinople, itself!
The two “halves” of the Church hadn’t been getting along, anyway– the Patriarchs had excommunicated each other in 1054–and having a western Christian army sack the Capital of the eastern Christian empire sealed a division that lasted into our own day.