What are the main differences between the Eastern (Orthodox) and Western (Catholic) churches?
The main theological difference between East and West, at least initially, concerns that from which the Holy Spirit proceeds. The West says that the Holy Spirit proceeds from the Father and the Son (filioque), whereas the East says that the Holy Spirit proceeds from the Father alone. This is sometimes referred to as “the filioque controversy”.Filioque, literally "and [from] the Son" is a Latin term that at one point was added to the Niceno-Constantinopolitan Creed (see below), not appearing in the original version.It probably appeared in the 6th century, but was accepted by the popes only in 1014. It is rejected by the Eastern Orthodox Church, Oriental Orthodox Churches and Church of the East. It is not in the original text of this Creed, attributed to the second ecumenical council (Constantinople I, 381), which says that the Holy Spirit proceeds "from the Father", without additions of any kind.As I mentioned before, this ONE WORD has been the subject of great controversy between Eastern and Western Christianity. It describes the double procession of the Holy Spirit and is translated into the English clause "… and the Son" in that creed:We believe in the Holy Spirit, the Lord, the giver of life,who proceeds from the Father and the Son.With the Father and the Son he is worshiped and glorified.The insertion of the filioque clause into the Nicene Creed by the Western patriarch in 1014, together with disputes over conflicting claims of jurisdiction (in particular over papal authority (Pope Leo IX claimed he held authority over the four Eastern patriarchs) were the main factors behind the East–West Schism.But… the Schism took place in 1054!! - And, since then (a loooong time ago), the Catholic and Orthodox churches have evolved differently, and a lot of other (very probably minor?) differences have appeared, some theological, many more in reference to their particular rites. They are too many to list here, but you can check the details in the following links:Eastern Orthodox – Roman Catholic theological differencesORTHODOXY AND ROMAN CATHOLICISMThe differences between the Catholic and Orthodox churchesDifferences & Similarities Between Eastern Orthodox Catholics & Roman Catholics