Other forms: missionaries
A missionary is someone who travels to a foreign country to perform charitable work and, most commonly, to try to convert people to their faith.
Missionary can be a noun — the person who goes on a mission — or an adjective — the type of work done on such a trip. If your great-grandmother was a missionary for the Methodist church in Japan in the 1920s, her purpose there was to convince people to abandon their religion for Methodism. Presumably she did this missionary work because she thought it would save their souls.