He arrived in Walvis Bay on 4 January 1845, and in Otjikango (Gross Barmen) on 9 April that year to work as an assistant to his fellow Rhenish Missionary Carl Hugo Hahn.
At the end of 1845, a major drought led to famine and the community was cut off from Walvis Bay, and Rath was assigned to go there by way of Windhoek to procure the necessary supplies.
He set out on 28 January 1846, three days before his 30th birthday, but found no passage to Walvis Bay available from Windhoek and had to travel on to Cape Town.
On 21 March that year, he married Anna Jörris van Mettmann of Düsseldorf (Germany) in Walvis Bay.
The nomadic Herero chief Zeraura agreed to settle there, leading the first of his people there with the rains in January 1850.
Rath built a simple brick house for himself and his daughters and a rudimentary building with holes for doors and windows to serve as the local church and school.
At the end of 1852, the Herero's perennial enemies, the Nama people, attacked Otjimbingwe, and after their repulsion Rath journeyed to Cape Town once more in January 1853.
Hahn wrote of the perilous journey in the RMS journal Berichte der Rhenischen Missionsgesellschaft (1858), in Petermanns Geographische Mitteilungen (nos.
11–17), and in a joint report of Rath, Hahn, and Green published under the title "Account of an expedition from Damara Land to the Ovampo in search of the river Cunene" (Proceedings of the Royal Geographical Society, vol.
Adinda Vermaak's 2004 book Kroniek van ’n kontrei, die verhaal van die NG kerk Kuilsrivier ("Chronicle of a Parish: The story of the Dutch Reformed Church (NGK) Kuilsrivier") recounts the tragedy as follows: In 1859, the Rev.
On 25 March 1859 the Rath family boarded the freighter Flora in Cape Town, en route to the mission in Damaraland.
Later, he heard from Katuti that - shortly after Leopold died - the rope Anna was holding slipped out of her hands, when she sunk into the icy waters.
After a while, stunned by the crashing waves, he focused on saving his own life and encouraged the still-living Katuti to climb higher.
(Matthew 27:45).Because his health had been affected by the shipwreck and he wanted to live closer to his two surviving children, he requested a transfer to the Rhenish mission at Sarepta near Kuils River.
Her sister, Emma, married a farmer from Kuils River, Daniël Francois (grandfather Danie) de Villiers and settled on his farm, Annandale.
Local farmers and other community members each donated 500 or more bricks so the school, consisting of two classrooms, could open on Easter Monday, 30 March 1891.
After retiring, Rath initially lived in Stellenbosch, but after the marriage in November 1898 of his daughter Anna (Otjimbingwe, 29 March 1851 – Wellington, 26 January 1940) to the Rev.
Rath and Hahn co-wrote the first hymns, scriptures, and Bible stories published in Herero (Omahungi oa embo ra lehova na Omaimpuriro mo Otjiherero, Cape Town, Saul Solomon & Kie, 1849).
The Grey collection at the National Library of South Africa in Cape Town features, among others, a short English-Herero glossary (from roughly 1865); an 1865 manuscript, Materialien zu einem Otjiherero-Deutschen Wöterbuche, in 17 notebooks, and an 1873 manuscript written in five notebooks: Otjiherero Wörtersammlung als Deutsches alphabethisches Register zu dem Otjiherero-Deutschen Wörterbuche von [J. Rath].