Bishop Theophilus AD 354
Bishop Theophilos was a native of Maldive Islands, off Kerala coast. Emperor Constantine took him as a hostage so that the Maldive people will not plunder Roman ships as it passed that way. In Rome he became a Christian and became a Bishop. He visited India and noted that their worship practices differed considerably from those of other parts of the world. Particularly he noticed that Indians sang, heard the gospel and worshipped sitting down (which is the Hindu tradition) he thought they were outrageous and ordered it changed. Probably the practice of worship standing was introduced from that time onwards.
Fr.Daniel AD 425
It may be assumed that Indians sent their priests for training and studies to Syria. There was one Daniel who translated the commentary on the Epistle to the Romans from Greek to Syriac in Edessa. He signed it as Daniel, the priest, the Indian. Ecclesiastical language of India was probably Greek and Syriac as the teaching of Bible came from there. Greek inscriptions are found on the bells of several churches. Until very recently the liturgy was mainly in Syriac. We maintain the flavor of this liturgy even today by retaining several Syriac phrases like Amen, Kurialaison, Brakomor,Sthaumenkalos et. and several Syrian chants.
Cosmos AD 522
Around AD 522, Cosmas Indicopleustes,a rich Christian merchant from Alexandria visited this coast. He says that in "Male" where the pepper grows, there are Christians and that at "Kalliana" there is a bishop, usually ordained in Persia. It is supposed that Male here means Malabar and Kalliana seems to be not Quilon but Kalyan near Bombay, but in order to form an opinion it is necessary to read his book called "Universal Christian Topology".He describes his visit: "We have found the church not destroyed, but very widely diffused and the whole world filled with the doctrine of Christ, which is being day by day propagated and the Gospel preached over the whole earth. This I have seen with my own eyes in many places and have heard narrated by others. I as a witness of truth relate: In the land of Taprobane (Srilanka), Inner India, where the Indian sea is, there is a church of Christians, with clergy and congregation of believers, though I know not if there be any Christians further in this direction. And such also is the case in the land called Male (Malabar), where the pepper grows. And in the place called Kallia (Kollam) there is a bishop appointed from Persia, as well as in the island called Dioscores (Socotra) in the same Indian Sea. The inhabitants of that island speak Greek, having been originally settled there by Ptolemies, who ruled after Alexander of Macedonia. There are clergy there also ordained and sent from Persia to minister among the people of the island, and the multitude of Christians...."