|Abdul Haris Nasution, General of the Indonesian Army|
Some Mandailing had previously converted to Islam, but the Padri war was a watershed event, with the Padri Wahabbis suppressing traditional 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.
Abdul Haris Nasution, General of the Indonesian Army
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 incompatbile 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, and the missionaries abandoned them as 'unreachable people', moving north to evangelize the Toba.
At the turn of the 20th century, nearly all Mandailing and Angkola were Muslims. Despite this, the Dutch administration them as part of the Bataklanden, and therefore heathen or Christian. This perception was an inaccurate one, and many Mandailing strongly rejected the 'Batak' label.
Abdullah Lubis, writing in the 1920s, claimed that while the Mandailing followed Batak marga practice, they had never followed the Batak religion, and that the Mandailing people pre-dated the Toba, having acquired marga directly from 'Hindu' visitors. In the Dutch census, the Mandailing objected strongly to being listed in the census as 'Batak Mandailing'.
Mandailing in Malaysia (who migrated in the years following the Padri war), had no such objection to their being deemed 'Malays', and indeed Malaysian Mandailing retain little of their distinct identity, partly due to a British colonial policy of rice-land ownership restrictions for all but Malay-speaking Muslims, and the disapproval of 'Batak' Muslim practices by the existing Malay Muslim population.
Modern Batak people are subject to religious influences through marriage and migration. Formerly Christian villages may, through migration of Muslim outsiders, lose their explicitly Christian identity, with pork excluded from wedding feasts in favour of meats acceptable to Islam.