In this third and final article on the Doopsgezind involvement with the atrocities of slavery in the former Dutch East and West Indies, the spotlight is exclusively on Marten Douwes Teenstra (1795-1864) from the northern Dutch Province of Groningen. Although born and raised in a Doopsgezind family and culture, Teenstra was in fact a marginal Mennonite, who also cherished Freemasonry, a phenomenon which was not uncommon in the nineteenth-century world of well-off Doopsgezind liberalism. Teenstra’s yet unknown role as an abolitionist ctivist has recently been revealed in a 2023 case study by Jaap Tuma. Taking his findings as a sound foundation, this article investigates why and how this somewhat odd individual and ex-farmer from rural Ulrum became actively involved with Dutch abolitionism. In the Netherlands, the anti-slavery movement gained little support, unlike England, for instance. Teenstra was the first person ever to send petitions to the Dutch Parliament in favor of abolition and emancipation, and he continued to do so for almost the next twenty years! After several unsuccessful attempts he finally gained success with the help of a liberal member of Parliament, his co-religionist Steven Blaupot ten Cate, also from Groningen. Since Teenstra’s initial role as an agitator in politics in The Hague, he soon evolved into a prominent opinion leader. No doubt together with two or three other abolitionist contemporaries, Teenstra should be considered an important and unprecedented forerunner for and promotor of antislavery, equal rights and antiracism in Dutch history.