Besides being a pious and a teacher, Tuan Guru Mustafa is also known as a reliable organizer. After returning from Mecca, he joined various Islamic social organizations. In 1915, he sat as chairman of the Tano Bato Islamic Society. In 1930 the Tapanuli Muslim Association (PMT) was established, and Tuan Guru sat as Chair of the Syar’iy Council. In 1936, he was entrusted to become the Advisor to the Executive Board of Al-Jami’atul Washliyah.
Tuan Guru established Al-Ittihadul Islamiyah (AII) to gather all Sikola Purba alumni scattered throughout the country. In addition, the Majlis Islam Tinggi (MIT) was found in Padangsidimpuan, and Tuan Guru was appointed to be its chairman.
One thing is interesting; after reading the text of the proclamation of August 17, 1945, the Islamic leaders in Tapanuli intend to carry out the tabligh akbar as an expression of gratitude for the independence that has been achieved. Ali Nuddin Lubis, the initiator, on the recommendation of Tuan Guru Mustafa, held the event in Panyabungan. The event was a great success. Therefore, the event developed into a Muslim Congress in Tapanuli. In 1947 the second Muslim Congress in Tapanuli took place and was attended by scholars from North Sumatra. At the congress, Tuan Guru decided that NU was the most appropriate organization for adherents of Ahlussunah waljamaah (Aswaja) understanding.
In 1950 the NU conference was held in Padangsidimpuan City. The conference was attended by NU administrators from both the regions and the central board. Inevitably, Tuan Guru was appointed Rois Syuriah NU Tapanuli. The teacher once said: “After your father investigated in depth the condition of the many Islamic parties, your father believed that the Nahdlatul Ulama party was a good one for your father to enter and all my children and my father’s followers would follow.”