You learn how to live out there, how to read a river, where the sand bar is, what the ripples mean. You learn how and where to fish. Before arriving in the jungle, missionary families generally learn Spanish in Costa Rica for a year. On the river, Spanish is a second language after local tribal dialects, and although locals have been given Bibles, most cannot read them. That angers missionaries who feel the nudity is immoral.
Anthropologists say evangelism threatens local customs and cultures.
There ain't nothing new under the sun. They are blaming the missionaries for the disappearance of native people. The people are disappearing because they are self-destructing.