IPA: /ɪnˈdɒktrɪneɪtə/
KK: /ɪnˈdɑktrɪˌneɪtər/
A person who teaches or influences others to accept a particular set of beliefs or ideas, often without questioning them.
The indoctrinator led the group in discussions about their core beliefs.
Indoctrinator → It is formed from "in-" (meaning into) and "doctrina" (from Latin, meaning teaching or doctrine) and "-tor" (a suffix indicating an agent or doer). The word refers to someone who teaches or instills a particular doctrine or belief into others.
Think of someone who is putting a specific 'teaching' ('doctrina') 'into' ('in-') others' minds — that's what an indoctrinator does.