I'm wondering if there is a definitive answer to this question. I was raised in the orthodox church and so I was always taught that Catholics split from the orthodox church and that it was the orthodox church which "retained the true faith and practices". Of course, once I grew up and left home...every catholic I met swore that it was the orthodox who did the splitting, and that it was the Catholics who retained the true faith and practices.
I don't understand why this question is so controversial. It seems by examining the practices of the church in the centuries before the great schism, it should be easy to tell which group deviated more and thus caused the split.
Well, it's kind of a tricky question, because there are essentially three reasons behind the split: 1) the Filioque, which was an addition to the Nicene Creed made by some bishops in the 6th Century in Spain, that later came to be accepted by the Latin Church, but was utterly rejected by the Orthodox Church for two reasons: a) it lacked precedent in the Scriptures, and b) it was done unilaterally without an Ecumenical Council; 2) the Catholic Church in the West had become consolidated around the Roman Curia and the Bishop of Rome. The rise of Secular Papal authority began with Gregory the Great, but did not reach its more modern levels until after the Investiture Controversy, in which the Pope superseded the authority of the Holy Roman Emperor. Additionally, due to the rise of Islam, what was originally the Pentarchy of Patriarchates (Rome, Alexandria, Antioch, Jerusalem, and Constantinople), was reduced to just Constantinople and Rome. The Bishop of Rome appealed to alleged scriptural authority of Rome being founded by St. Peter and St. Paul, but the Orthodox Church opposed this because it lacked precedent and it was interpreted as being a fundamental alteration of the East-West Dynamic. Originally, the East was far more populous and had far more Christians than the West, but in the 11th Century, that situation had completely reversed and the Catholic Church found itself paying direct obedience to the Pope as the head of the Church. Lastly, the final reason for the schism was the emergence of different trends in liturgical rites. The East used the Byzantine Rite, using leavened bread and the Divine Liturgy of St. John Chyrosostom, and the West used the Roman Rite, which used unleavened bread and the Gregorian Liturgies. Now, normally that wouldn't be so much of an issue, as the Irish and French maintained their own liturgical rites, but it was viewed as being a source of pride for the Byzantines to have uniformity within the Church. Thus, since the West had deviated from what the Greeks considered the norm, they were turning their back on the Byzantine traditions.