of church activities as a result of thousands of foreign missionaries being interned or forced to leave after the 1942 invasion of Sumatra by the Japanese. The Gereja Kristen Protestan Simalungun, originally part of the HKBP and preaching in Batak Toba language, eventually became a distinctively Simalungun church, adopting Simalungun customs and language, before finally incorporating as GKPS in 1963. Islam The Mandailing and Angkola people, occupying the southern Batak lands, came under the influence of the neighbouring Islamic Minangkabau people as a result of the Padri War (1821–1837). Some Mandailing had previously converted to Islam, but the Padri war was a watershed event, with the Padris suppressing traditional customs (adat) and promoting 'pure' Islamic faith. Over time Mandailing Islam, has been brought closer to the predominant Southeastern Shafi`i school of Islam as a result of Mandailing discourse with other Islamic practitioners and the practice of hajj, although traditional elements remain, such as dividing inheritance among all children, a Mandailing rather than Islamic practice. Islam caused the decline in importance of marga, with many Mandailing abandoning their marga in favour of Muslim names, much less so among the Angkola to their North. The advent of Islam also caused the relegation of the datuk to a medicine man, with traditional rice-planting ceremonies and other such remnants of traditional culture deemed incompatible with Islam. The 'pasusur begu', a ceremony invoking ancestors to aid the community, was also suppressed. Other aspects of adat were however tolerated, with the Mandailing Islamic ideology placing adat on the same level as Islamic law, as in contrast with the Minang practice of placing Islamic law above adat. In more recent times, learned Islamic scholars (ulama) studying abroad, have suggested that many traditional Mandailing practices, such as the 'Raja' hereditary leaders, were in conflict with Islam, being indicative of 'pele begu'. The Islamist ulama were in conflict for authority with the Namora-Natora, the traditional village legal practitioners, who were influenced by adat as much as Islam. Christian missionaries had been active among the northern Mandailing from 1834 onwards, but their progress was restricted by the Dutch government, who feared conflict between newly converted Christians and Muslims. In addition, the lingua franca of the government was Malay, associated with Muslims, as were government civil servants, creating the perception that Islam was the religion of modernity and progress. Missionaries determined that resistance among the Muslim Mandailing to Christianity was strong,