Mar Rowil (Reuben) Shimun, Catholicos Patriarch, XX, the twentieth patriarch in the family's succession was born in Qudchanis, located in South East Turkey. We believe the year of his birth to be 1844. He assumed the mantle of Patriarchal responsibility in 1860 thru 1903, encompassing forty-three years serving Assyrians and the millat (nation).

Mar Rowil was loyal to the Turkish government. In turn, the Patriarch had a direct communication path to the Turkish Sultan on any matter he considered important. Illustrative of the relationship and trust, the Turkish government honored the Patriarch with two significant medals, the Order of the Osmanieh and the Order of the Mejedie. However, with the arrival of the missionaries came massacres committed by the Kurdish Emirs, who believed the Assyrians, as Christians, were aligned with the western missionaries and the governments they represented. It was further believed by the Kurds that there was an interest in controlling the Hakkari region, by leveraging the help of the Western missionaries. It was during this time that hatred and animosity between the Kurds and the Assyrians commenced.

The Turkish government proposed that Mar Rowil act as the spiritual leader, and that his father, Benyamin, act as the political leader thereby separating the two offices due to the enormity required of both sets of responsibilities. Mar Rowil consulted with the bishops and tribal leaders who were not pleased with this proposal. They felt this was suspect and advised the Patriarch this was a trick planned by the government, and believed that after the death of the Patriarch's father, the Turkish government would usurp political responsibility and appoint a non-Assyrian in his place. The government accepted the decision and Mar Rowil was afforded the respect and trust by the local government.

In 1903, Mar Rowil Shimun passed away and was interred in the Church of Mar Shalita, which was built by Mar Dinkha Shimun XV. Seven Patriarchs from the d'Mar Shimun family have been laid to rest in the Church, the last of which was Mar Rowil.