The appointment comes amid broader concerns about the future of Brazil’s Indigenous communities.

President Jair Bolsonaro has long been critical of the policy of setting aside vast territories for Indigenous groups, calling it an impediment to economic growth. His administration is seeking to create a legal framework that would allow mining ventures in some of those lands.

He has also compared indigenous communities living in remote areas to animals in a zoo.

The most vulnerable of Brazil’s Indigenous communities are the groups — which by some estimates number more than 100 — that have had little or no contact with the outside world. The National Indian Foundation has confirmed sightings of approximately 28 such communities, and provides health care and guidance to about 11 of them that have recently chosen to emerge from total isolation.

The missionary group Mr. Dias worked for from 1997 to 2007, which was called New Tribes Mission at the time and is now known as Ethnos360, argued that there is an urgent need to convert all tribes that have not been exposed to “the Gospel of Christ” in order to save them from “unrelenting spiritual darkness.”

“I’ve been in many of these tribes and at times you can feel this incredible and tense darkness,” Larry Brown, the group’s chief executive, said in a video posted on its website. “But you know what I found: No darkness is too dark for God.”

Leila Sílvia Burger Sotto-Maior, an anthropologist who retired from the National Indian Foundation in 2018, said there is deep alarm among her former colleagues at the agency about the fate of uncontacted and newly contacted tribes under the current government.

The agency, known as FUNAI, has been hit with budget cuts that have sharply limited its ability to monitor indigenous territories that have been invaded by wildcat miners, farmers and loggers.

“There is a sense that a policy that was built over so many years, and was working, is now being dismantled,” she said. “There’s a sense of hopelessness.”