After moving to Bandung from his hometown Solok, West Sumatra for senior high school, Natsir studied Islamic doctrine extensively.

After his term as prime minister, he became increasingly vocal about Islam's role in Indonesia and was eventually arrested for doing so.

He viewed Islam as an intrinsic part of Indonesian culture and was disappointed by the Sukarno and Suharto government's handling of the religion.

In 1923, he continued his studies at MULO (Meer Uitgebreid Lager Onderwijs) and he joined Pandu Nationale Islamietische Pavinderij and Jong Islamieten Bond.

[12] After his release from prison, Natsir became increasingly involved with organizations related to Islam, including the Majlis Ta'sisi Rabitah Alam Islami and Majlis Ala al-Alami lil Masjid, both in Mecca, the Oxford Centre for Islamic Studies in England, and the World Muslim Congress in Karachi, Pakistan.

He also criticized government policy, like when he signed the Petisi 50 (Petition of Fifty) on 5 May 1980, which caused him to be banned from going overseas.

[17] Unlike the secular, pro-communist Sukarno, who viewed religion as an entity separated from the nation, Natsir believed that the separation of church and state did not apply to Indonesia, as he saw it as an intrinsic part of their culture and one of the main reasons they fought for independence.

To support his position, he often quoted William Montgomery Watt, saying that Islam is not just a religion, but an entire culture.

[17] Natsir published 45 books or monographs and several hundred articles dealing with his views of Islam.

[19] His later works included some written in English[18] and focused more on politics, as well as the preaching of Islam and Christian-Muslim relations.

[20] Ajip Rosidi and Haji Abdul Malik Karim Amrullah have noted that Natsir's writings serve both as historical records and also as guides for future Muslims.

[13] Natsir could reportedly speak numerous languages, including English, Dutch, French, German, and Arabic; he was also capable of understanding Esperanto.