From Lexicanum

A Missionary is tasked with spreading the word of the Imperial Creed throughout the Imperium. They have little guidance from the Adeptus Ministorum and are mainly free to spread the faith in any way they see fit.

They usually run charitable institutions called Missions on newly discovered human words. Missions take the form of schools and hospitals. One of the purposes of these institutions is to further the observance of the Imperial Cult. The most famous Missions are the Schola Progenium^1,2.

Missionaries range from the young fervent hopefuls to the old, wise, and toughened members of the Missionarus Galaxia. They are always at the forefront of Imperial expansion and accompany the crusading armies as they discover new worlds and contact lost civilizations.

Sources

• ¹Codex Imperialis, Rick Priestley
• ²Commissar Training Squads, White Dwarf/Warhammer 40,000 Compilation, 1991