August Theis led a group of German missionaries to spread Christianity to the Simalungun area after he received a telegram from the local Rheinische Missionsgesellschaft leader. On September 2, 1903 they arrived at Pematang Raya to begin their mission among the Simalungunese. September 2 has been celebrated each year by GKPS all over the world as an olob-olob day to thank God that the Bible has entered Simalungun. The sending mission was effectively started in 1904 with August Theis at Pematang Raya and Guillaume at Purba Saribu. Not until 1909 did the first Simalungunese Christian baptism occur at Pematang Raya. Later in the same year, 38 other Simalungunese were baptized at Parapat. Up to 1910, 17 churches were founded inside the Simalungun area. They were:
The first missions were held using the Toba language. Simalungunese resistance to the Westerners and their lack of comprehension of the Toba language decreased RMG effectiveness and therefore held back the growth of Christianity among the Simalungunese. On September 1, 1928, the celebration of the 25th anniversary of Christianity at Simalungun was held at Pematang Raya. On this occasion, some Guru and Sintua agreed to found a committee which planned to translate church-related text books into the Simalungun language. This produced Haleluya and Manna. On November 15 of the same year, a group called "Kongsi Laita" was founded at Sondiraya by some members of the Pematang Raya church to ask the Simalungunese to spread Christianity by themselves to others.
On September 26, 1940, all of the churches inside the Simalungun area were officially grouped together as a single district inside HKBP. 12 years later on October 5, 1952, Simalungun District synod members held a meeting to prepare a separation between themselves and HKBP. The new church was called HKBPS. HKBPS reorganized themselves on November 30, 1952, by dividing themselves into three districts. An administrative office was also built in Sudirman Street, Pematang Siantar, as a headquarters.
HKBP-Simalungun transformation into GKPS
HKBPS transformed its organization and name into Gereja Kristen Protestan Simalungun on September 1, 1963. In order to serve the Simalungunese better, GKPS founded its educational center at Pematang Raya and a dormitory for boys and girls. GKPS also founded a centre for agricultural training on January 15, 1964, at Pematang Siantar, which is called PELPEM GKPS.
