According to Wikipedia, the two most spoken indigenous languages on Pohnpei are written with the Latin alphabet.
https://en.m.wikipedia.org/wiki/Pohnpeian_language
https://en.m.wikipedia.org/wiki/Chuukese_language
See the "phonology" and "orthography" sections of the respective articles. It seems like both have a standard orthography.
Very interesting.
Perhaps my family member confused the fact that the religion had no training materials for the language with there being no written version of the language. (I'm admittedly ignorant about this, I just had a conversation with them on Saturday)
I wonder if the language isn't as commonly written as it is spoken?