Sleeba Mar Osthathios was born in the village, Kafarso, (near Shrehre) near the city of Amid, south-east Turkey on 1 January 1854. He was born in the Abakkar family to Salbi and Naboona. He had two brothers and three sisters. His father Salbi died when Sleeba (Saliba) was 3 years old. A few years later his two brothers & two sisters also died of a deadly disease. However, Saliba, his mother & one sister survived the epidemic.
His father’s brother, Priest Dhenha, was his first teacher. Later he learned Syriac and theology from very Rev. Samuel Ramban who was the vicar of the Damascus Church. In 1849, Very Rev. Samuel Ramban came to Malankara (Kerala, India) with Mar Athanasius Stephanos at the direction of Mar Yacoub II, the Patriarch of Antioch. Ramban Samuel returned home after eight years. At that time, the protestant missionaries were causing trouble in the Church.
Mar Patriarch Peter III came to India in 1875, to sort out the creeping influence of the Protestant reformation movement sponsored by the British CMS. In 1880, the Patriarch Peter III ordained Samuel Ramban Bishop by name Athanasius Shem’vun. In the same year Sleeba was ordained Kooroyo by Mar Athanasius Shemavun. They travelled to Malankara in January of 1881. Mar Athanasius Shem’vun and Deacon Sleeba stayed at Puthenpally, Kottayam. In 1889, the Court Order came in favour of Pulikkottil Joseph Mar Dionysius. Subsequently, Mar Athanasius Shem’vun and Deacon Sleeba moved to Old Seminary, Kottayam.
On 11 June 1889 Mar Athanasius Shem’vun departed for his heavenly abode while staying at Old Seminary, Kottayam. The mortal remains were interred at Puthenpally, Kottayam. This was indeed saddening for Deacon Sleeba, when he lost his teacher in India, but was not discouraged at the loss, instead continued to work in the vineyard of the Lord.
Mar Gregorius Geevarghese (Chathuruthil) had a special affection for Deacon Sleeba and Deacon also formed a close fraternity with St. Gregorios Geevarghese of Parumala. Deacon Sleeba visited Mar Gregorius at Parumala on several occasions. In 1895, Mar Gregorius visited the Holy Land for which all the arrangements were made by Deacon Sleeba and he accompanied the Bishop on his visit and provided great assistance during the pilgrimage. Mar Gregorius departed for the eternal rest, on 2 November 1902. Deacon Sleeba again felt a big vacuum, but he continued with the gospel work. He learned to speak in Malayalam, though with a foreign accent.
In1906, Deacon Sleeba returned to Mosul, Iraq along with Deacon Mathew (Chettalathumkara) who later became Catholicos Augen. Both of them stayed at the monastery of Mar Mathai, Mosul. Mar Coorilos Elias Kudso, the Reesh Dayro, ordained Deacon Sleeba as Ramban. After two weeks Ramban Sleeba was ordained Priest. In March 1908, Patriarch Abded Aloho II ordained Ramban Sleeba as Bishop by name Osthathios at the Church of Forty Martyrs of Sebaste, at Mosul and send him to India.
Mar Dionysius Joseph was in his old age and Mar Ivanios Murimattathil retired from his administrative duties. In these circumstances in 1908 the meeting of the representatives of the Church met and elected priest Geevarghese (Vattasseril) and Ramban Paulose (Kochuparambil) to be ordained Metropolitans. They went to Mosul and the Patriarch Abded Aloho II ordained them Metropolitans at Jerusalem. The three newly ordained Bishops, Mar Dionysius Geevarghese Vattasseril and Coorilos Paulose Kochuparambil, and Sleeba Mar Osthathios, returned to Malankara along with Ramban Augen.
Mar Osthathios remained devoted to prayer & fasting, his saintliness being observed by all whom he encountered. He was a saintly figure. His books originally written in Syriac were translated into Malayalam. After serving the Holy Church for decades, Mar Osthathios Sleebo reposed in the Lord on March 19th, 1930, and his mortal remains were interred at St. Mary’s Puthen Palli Arthat, Kunamkulam.