Claiming more than 500,000 members,[1] the SIB is the largest Protestant denomination in Malaysia having evolved from a small missionary presence among the Lun Bawang people of the Kelabit Highlands.
Sarawak's border areas were exposed to influences from American missionaries in Kalimantan (then known as Dutch Borneo), and most of the Lun Bawangs became Christian through interaction with their kinfolk, indigenous converts on the other side of the international boundary.
The first missionaries to visit the Kelabit Highlands in the Sarawak interior arrived in 1939, but the internment of Europeans during World War II opened the door to indigenous leadership.
With the emphasis on training married couples to extend mission work and Kelabit out-migration to the oil-fields of Miri and elsewhere in search of employment and education, the SIB church spread through Sarawak.
In line with the emphasis of the 'Twin Primary Tasks'[clarification needed] of BEM, SIB seeks to be significantly involved in evangelism and to build up churches through Bible teaching and leadership training.
The SIB is also well established in urban centres notably Lawas, Limbang, Miri, Kuching, Sandakan, Tawau, Labuan and Kota Kinabalu where many native villagers have migrated in search of work.
In the early 1990s, Pastor Richard Samporoh and late wife, Pastor Stemmah Sariau, native Dusun church members who are emigrants from Kota Belud in the northern part of Sabah's West Coast Division, started a SIB ministry in their house after noting the increasing number of Sabah and Sarawak natives emigrated to the Peninsular either for study or seeking employment.
The church's leadership was handed over to a married couple, Dr. Chew Weng Chee, an obstetrician-gynecologist from Tung Shin Hospital, and Lew Lee Choo, another doctor who studied in the UK.
Services are held in Malay, English, Chinese and native ethnic languages including Iban, Bidayuh, Kadazandusun, Kayan, Kelabit, Kenyah, Lun Bawang, Penan, Punan, Tagal, Orang Asli, Melanau and several others.