You are going to find that depending on who you talk to, you will get a slightly different answer to your question. Here’s one view:
One of the main differences which caused the split was that the Roman Church added the filioque to the creed. The original Nicene Creed said that the Holy Spirit proceeds from the Father. This was agreed to by both the Eastern Churches and the Roman Church. Then, later on, without consulting with the Eastern Churches, the Roman Church decided it would unilaterally change the creed on its own and add the filioque to the creed. So the Roman Church says that the Holy Spirit proceeds from the Father and from the Son. It then demanded that the Eastern Church accept this addition, which the Eastern Church said it would not, because it had agreed to the creed without the filioque and the Roman Church had also agreed to the creed without the filioque and the Eastern Church said that it was not a correct teaching to add the filioque. Then in 1054, the papal legate Cardinal Humbertus placed a bull of excommunication on the altar of the Hagia Sophia citing the filioque as a reason for the excommunications and anathemas. There were other issues also mentioned, such as the question on the use of unleavened or leavened bread, etc.
In addition to this, there occurred later on a Crusade, known as the Fourth Crusade, which resulted in the incredible violence, looting, plunder of the Eastern Churches by the Catholic crusaders. For a description, please take a look at the article at Catholic Fordham University, Dana C. Munro, "The Fourth Crusade ",
fordham.edu/halsall/source/4cde.html#sack
" Then it was announced to all the host that all the Venetian and every one else should go and hear the sermons on Sunday morning; [Apr 11, 1204] and they did so. Then the bishops preached to the army, the bishop of Soissons, the bishop of Troyes, the bishop of Havestaist [Halberstadt] master Jean Faicette [De Noyon, chancellor of Baldwin of Flanders], and the abbot of Loos, and they showed to the pilgrims that the war was a righteous one; for the Greeks were traitors and murderers, and also disloyal, since they had murdered their rightful lord, and were worse than Jews. Moreover, the bishops said that, by the authority of God and in the name of the pope, they would absolve all who attacked the Greeks."…
" How shall I begin to tell of the deeds wrought by these nefarious men! Alas, the images, which ought to have been adored, were trodden under foot! Alas, the relics of the holy martyrs were thrown into unclean places! Then was seen what one shudders to hear, namely, the divine body and blood of Christ was spilled upon the ground or thrown about. They snatched the precious reliquaries, thrust into their bosoms the ornaments which these contained, and used the broken remnants for pans and drinking cups,-precursors of Anti-christ, authors and heralds of his nefarious deeds which we momentarily expect. Manifestly, indeed, by that race then, just as formerly, Christ was robbed and insulted and His garments were divided by lot; only one thing was lacking, that His side, pierced by a spear, should pour rivers of divine blood on the ground.
Nor can the violation of the Great Church [Hagia Sophia] be listened to with equanimity. For the sacred altar, formed of all kinds of precious materials and admired by the whole world, was broken into bits and distributed among the soldiers, as was all the other sacred wealth of so, great and infinite splendor.
When the sacred vases and utensils of unsurpassable art and grace and rare material, and the fine silver, wrought with go , which encircled the screen of the tribunal and the ambo, of admirable workmanship and the door and many other ornaments, were to be borne away booty, mules and saddled horses were led to the very sanctuary of t temple. Some of these which were unable to keep their footing the splendid and slippery pavement, were stabbed when they fell, that the sacred pavement was polluted with blood and filth.
- Nay more, a certain harlot, a sharer in their guilt, a minister the furies, a servant of the demons, a worker of incantations and poisonings, insulting Christ, sat in the patriarch’s seat, singing an obscene song and dancing frequently. Nor, indeed, were these crimes committed and others left undone, on the ground that these were of lesser guilt, the others of greater. But with one consent all the most heinous sins and crimes were committed by all with equal zeal. Could those, who showed so great madness against God Himself have spared the honorable matrons and maidens or the virgins consecrated to God?
Nothing was more difficult and laborious than to soften by prayers, to render benevolent, these wrathful barbarians, vomiting forth bile at every unpleasing word, so that nothing failed to inflame their fury. Whoever attempted it was derided as insane and a man of intemperate language. Often they drew their daggers against any one who opposed them at all or hindered their demands.
No one was without a share in the grief. In the alleys, in the streets, in the temples, complaints, weeping, lamentations, grief, t groaning of men, the shrieks of women, wounds, rape, captivity, t separation of those most closely united. Nobles wandered about ignominiously, those of venerable age in tears, the rich in poverty. Thus it was in the streets, on the corners, in the temple, in the dens, for no place remained unassailed or defended the suppliants. All places everywhere were filled full of all kinds of crime. . Oh, immortal God, how great the afflictions of the men, how great the distress!"
" How shall I begin to tell of the deeds wrought by these nefarious men! Alas, the images, which ought to have been adored, were trodden under foot! Alas, the relics of the holy martyrs were thrown into unclean places! Then was seen what one shudders to hear, namely, the divine body and blood of Christ was spilled upon the ground or thrown about. They snatched the precious reliquaries, thrust into their bosoms the ornaments which these contained, and used the broken remnants for pans and drinking cups,-precursors of Anti-christ, authors and heralds of his nefarious deeds which we momentarily expect. Manifestly, indeed, by that race then, just as formerly, Christ was robbed and insulted and His garments were divided by lot; only one thing was lacking, that His side, pierced by a spear, should pour rivers of divine blood on the ground. "
