Under the Federal Constitution, there would be no such thing as "Malay Christian" as a "Malay" must be a Muslim. If a Malay ceased to be a Muslim, he would no longer be a "Malay" and lost his special rights as a Malay.
The reasons why Malay Language is used in the churches:
1) Most of the younger generation of non-Muslim natives of Borneo (ie. Iban, Kadazan, Murut, Kelabit, Dusun etc) speak Malay Language as first language.
2) The Christian Aborigines of Peninsula Malaysia could only read the Bible in Malay Language as their native languages have no writing systems and Malay Language is the only foreign language they knew.
3) The Christian Indonesian immigrants could only understand Malay Language as there are no Indonesian Language churches in Malaysia.