However, I never knew that before the brutal Portuguese invasions, all Christians in India were members of the Church of the East! That came as quite a shock to me. Previously, I had figured that the Oriental Orthodox Church and Church of the East sort of did their own thing and converted the people of the Malabar Coast (and beyond, even into northern Sumatra if Wikipedia is to be believed) on their and co-existed.
Actually what you had figured is not completely incorrect. When people think of the Church of the East they immediately think of the Nestorian (though there are claims that this church did not hold on to the Nestorian heresy as others accuse them of) church. The fact is there was an autonomous Oriental Orthodox “Church of the East” right from the time of Ephesus. The head of the Oriental Orthodox Church of East was called Maphrian http://en.wikipedia.org/wiki/Maphrian
to distinguish him from the head of the Nestorian Church of the East; Catholicose.
Both Oriental Orthodox Church of the East as well as the Nestorian Church of the East did their own thing in Malabar Coast.
There is surviving pre-Protughese inscription on a church wall that refer to Virgin Mary as Yaldath Aloho aka Theotokos. This clearly had to be Oriental Orthodox as the Nestorian Church would not have used that term.
*How did the Nestorian community's hierarchy fall apart?
Around the same time period as the arrival of the Portuguese to India, there was something interesting happening in the Nestorian Church of the East. This church had a split over who gets to be the Patriarch-Catholicose. This was in the 1553. One of the groups joined with Rome and became a “Rite” under the Pope. This group survives to this day and is called the Chaldean Catholic Church. The Nestorian Church of the East that did not join with Rome is today known as the Assyrian Church of the East.
Portuguese landed on the Malabar coast in 1498 and by the 1550s was the predominant Naval Power on the west coast of India. They pretty much controlled all the ports on the Malabar coast and controlled export / import of goods as well as what kind of Christian hierarchs could come and go.
After the 1553 split in the Nestorian Church of the East; only the Roman Rite ; ie the Chaldean Catholic Church was successful in sending Bishops to India. No Bishop from the Assyrian Church of the East or the OO Church of the East under the Maphrian was successful in landing in India from the time of the arrival of the Portughese till they were defeated and over powered by the Dutch.
The last two Mesopotamian Bishops in India are:
Mar Joseph http://en.wikipedia.org/wiki/Joseph_Sulaqa
(He is the brother of John Sulaqa the first Patriarch of the Chaldean Catholic Rite ; and the leader of the group that joined with Rome)
Mar Abraham http://en.wikipedia.org/wiki/Abraham_Methran
Mar Abraham died in 1597
The Synod of Diamper ; the church council conducted by the Portughese that formally brought the Malabar Church under Rome happened only in 1599. But since Mar Joseph and Mar Abraham were also in communion with Rome via the Chaldean Catholic Rite and they served as prelates since 1556.. one can argue 1556 as the year in which the Malabar church came under Rome.
*How did the Oriental Orthodox Church move in to save the Saint Thomas Christians from joining the Catholic Church after their own Church had fallen apart?
For 54 years, from 1599 to 1653, the church in Malabar was under Rome led by Portuguese Prelates. Also by 1650s Portuguese power on the west coast of India had declined. Dutch was the predominant naval power. In 1653, several priests and lay members gained courage and they assembled at Mattanchery and took an oath touching on a cross (since everyone could not touch the cross, they tied a long rope to the cross, and everyone touched the rope). In this oath known as the Coonen Cross Oath http://en.wikipedia.org/wiki/Coonan_Cross_Oath
, the Malabar Church declared its independence.
The oath was: By the Father, Son and Holy Spirit that henceforth we would not adhere to the Franks, nor accept the faith of the Pope of Rome.
And after taking the oath, on 22-May-1653, 12 priests laid their hands on one of the priests and elevated him as Bishop with the title Mar Thoma I.
As soon as these events took place in India, Rome took notice. One of the 12 priests who ordained Mar Thoma I , went back from the oath.
This priest Fr. Parambil Chandy , went to Rome and was ordained by Rome as a Bishop with the title Alexander de Campo. Rome also brought back the Syriac Liturgy. But this was the Latin Mass translated to Syriac and not the original Church of the East Syriac Liturgy.
Now there were two jurisdiction in the Malabar Church, one headed by Mar Thoma I and a RC rite.
There were doubts about the legitimacy of the ordination of Mar Thoma I as he was not properly ordained, but enthroned by 12 priests as a Bishop. So the Malabar Church appealed to the OO Syriac Orthodox, Patriarchate of Antioch and all the East for help. Keep in mind, the Syriac Orthodox Church had pre-Portughese connections with the Malabar Church via the Maphrian. Also by this time the Dutch had overtaken the Portughese as the predominant Naval Power on the west coast. The Protestant Dutch, didn’t care much about which Bishops could land on the Malabar coast. So it was once again possible for Non-RC affiliated Bishops to come to Malabar.
In 1665 Mor Gregorious Abdul, http://en.wikipedia.org/wiki/Gregorios_Abdul_Jaleel
the Syriac Orthodox Archbishop of Jerusalem was send to the Malabar coast by the Syriac Synod. Mor Gregorious regularized the episcopal ordination of Mar Thoma I who was ordained 2 years earlier by 12 priests.
Mor Gregorious insisted on the removal of all Nestorian references in the liturgy; and that can be referred to as the starting point of the adoption of the West Syriac liturgy in Malabar.
*Is there any record of there being difficulty with the transition for the Christian community of India from the Church of the East to the Oriental Orthodox Church?
None that I am aware of. We have to keep in mind that there was a pre-portughese OO presence in India.
Also right from the time of Mar Joseph 1556 there was no true Assyrian Church of the East presence in India. Mar Joseph was from the Chaldean Catholic Church an RC rite that was started by his bother Mar John Sulaqa joining with Rome.
And at 1665 there were only two jurisdictions; the RC rite that used the Syriac translation of the Latin Mass and the Syriac Orthodox under Mar Thoma I.
*Why did the Saint Thomas Christians switch from an East Syriac liturgy to a West Syriac liturgy? Was that change necessary, or did the community choose to make the shift? Are there significant differences between the two?
By 1665, there was no original Nestorian East Syriac liturgy left in Malabar. It had evolved into a latinized form of the East Syriac liturgy coming from the ‘Chaldean Catholic’ rite and later becoming a complete Syriac translation of the Latin Mass ; but pronounced in East Syriac.
So when the Malabar Church under Mar Thoma I got official recognition from the Syriac Orthodox Synod via Mar Gregorious Abdul (Syriac Orthodox Archbishop of Jerusalem), the Malabar Church started accepting the West Syriac liturgy of the Syriac Orthodox Church.