missionary; evangelist; propagandist
Refers to a person who actively spreads a religion, doctrine, or ideology. Can be used for both religious missionaries and secular propagandists.
He went to Africa as a Christian missionary.
その政治家は自らの思想の布教者として活動している。
The politician acts as a propagandist for his own ideas.
伝道師 is often used for Protestant evangelists or preachers, whereas 布教者 is a broader term.
Compound of 布教 (ふきょう, 'propagation of a religion or doctrine') + 者 (しゃ, 'person'). The exact historical derivation is uncertain, but the word follows a common pattern for agent nouns.