In 1876, Coolsma returned to the Netherlands and became the leader of the Netherlands Missionary Union, promoting further missionary activity in the predominantly Muslim western portion of the East Indies.
Pada tahun 1876, Coolsma pulang ke Belanda dan menjadi ketua Serikat Misionaris Belanda dan mempromosikan aktivitas misionaris di wilayah barat Hindia Belanda yang didominasi Muslim.