This is the year traditionally seen as the year of the Great Schism between Eastern Orthodox and Western Catholic Christianity. In truth, events unfolding after those of the year 1054 more complexly and slowly lead to the great break in the Church, but there were dramatic dividing events during 1054.
A man called Cardinal Humbert was legate to Pope Leo IX and in 1054 he was in Constantinople. He was not the most tactful diplomat and he and the Patriarch Michael Cerularios of Constantinople did not get along.
The problems had all started when Normans conquered southern Italy. This region had been under Byzantine control and held to the customs of the Greek church. The Normans started imposing western Latin customs on the church in southern Italy and suddenly everyone was worried about previously insignificant details like the type of bread used for the Eucharist. The Patriarch of Constantinople was not pleased about the Greek traditions being replaced by Latin ones in Southern Italy, so in anger he suppressed the Latin churches under his own control in Constantinople itself.
Then things got heated really heated in some letters sent between Pope Leo IX and Michael Cerularios. The pope was sending out letters to make sure it was remembered that Rome had highest authority in the Church but Michael Cerularios was incensed enough to write a letter back, calling Pope Leo IX ‘brother’ instead of ‘father’ and claiming that the patriarch of Constantinople was ‘Ecumenical Patriarch’, which was just getting a bit too big for his boots in the eyes of Leo IX.
So Cardinal Humbert was sent to deal with Michael Cerularios. And the men just did not like each other. So Humbert, infuriated, stuck a notice of excommunication on the door of the Hagia Sofia. It wasn’t valid, wasn’t signed by the pope (who had actually already died by that time). However, tensions did not truly cease from there and ultimately, the Eastern and Western church would split apart entirely.