(CNN) John Allen Chau was an adventurer who hiked in North Cascades National Park in the Pacific Northwest, traveled to Israel and went on mission trips to South Africa. But he was always drawn to North Sentinel Island off the coast of India, and the people there.

The Sentinelese live in isolation on the remote island in the Andaman and Nicobar archipelago, protected by Indian law to maintain their way of life and protect them from modern illnesses because they lack immunity. Chau took a scouting trip to the Andaman islands several years ago and told people of his desire to return, said a friend, John Middleton Ramsey.

Chau's zeal to spread the Christian gospel took him back to the remote island, where he apparently was killed last week by tribespeople after trespassing, authorities said. Contact with the isolated tribe is prohibited. But those who knew the American missionary are calling him a martyr for the Christian faith.

"He was someone who died out of love for these people to bring the good news of Jesus Christ," Ramsey, 22, said in an interview Wednesday from Cologne, Germany.

'We refuse to call him a tourist'

Indian authorities say Chau was 27, but Mat Staver, founder of a Christian ministry that Chau was involved with as a college student, gave Chau's age as 26.

Chau came to India on a tourist visa but traveled to the Andaman and Nicobar Islands in October to proselytize, according to Dependra Pathak, director general of police of the Andaman and Nicobar Islands.

"We refuse to call him a tourist. Yes, he came on a tourist visa, but he came with a specific purpose to preach on a prohibited island," he said.

Just more than a dozen people are believed to live on the island, which is a protected area. People are not allowed to go within 5 nautical miles of the island after previous instances of aggressive behavior toward outsiders. In 2006, the tribes killed two local fishermen.

Chau asked a local friend, an electronic engineer, to get a boat and find several fishermen and a water sports expert to help with the expedition, according to Pathak.

The fishermen said they used "a wooden boat fitted with motors to travel to the island on November 15," Pathak said.

The boat stopped a little less than half a mile away and Chau "used a canoe to reach the shore of the island," Pathak said. Chau returned later that day with arrow injuries. On November 16, "the (tribespeople) broke his canoe."

"So, he came back to the boat swimming. He did not come back on the 17th; the fishermen later saw the tribespeople dragging his body around," Pathak said.

Police haven't independently verified Chau's death, but they believe he was killed, based on the account of the fishermen. Authorities have yet to recover Chau's body.

'He loved Jesus'

Raised in Vancouver, Washington, Chau was first drawn to the outdoors after discovering a copy of "Robinson Crusoe" while in elementary school, he said in an article several years ago in The Outbound Collective , a website and app that helps people discover the outdoors.

He and his brother would paint their faces with wild blackberry juice and run around their backyard with bows and spears made from sticks, according to the article.

Chau graduated from Oral Roberts University, where he got involved with Covenant Journey, the Christian ministry that takes college students on immersion trips to Israel, according to Staver, who is the group's chairman.

Chau traveled to Israel with Covenant Journey, and to South Africa on missions with a group at Oral Roberts, Staver said.

"John loved people, and he loved Jesus. He was willing to give his life to share Jesus with the people on North Sentinel island," Staver said in a press release. "Ever since high school, John wanted to go to North Sentinel to share Jesus with this indigenous people."

In the Outward Collective article, Chau spoke of his earlier adventures, including hiking Table Mountain in Washington state on Christmas break while in college.

Chau said going back to the Andaman and Nicobar Islands was on the top of his adventure to-do list, the article said.

Chau knew the risks but it 'didn't frighten him'

Chau did not tell police of his intentions to travel to North Sentinel to attempt to convert its inhabitants, officials said. But he told a few people close to him, like Ramsey, who said Chau knew the island was a restricted area and his mission there was illegal.

Ramsey, who met Chau in August 2015 on a Covenant Journey trip to Israel, said they "both had a passion for sharing our faith with others."

When Chau returned from this scouting trip, Ramsey recalled Chau talked about his plans to return to the remote region, bearing gifts for the Sentinelese people. He said he wanted to get to know the islanders' way of life, eventually share the gospel and perhaps translate the Bible, Ramsey recalled.

"I kind of gathered that he would have been open to staying there for the rest of his life, but he didn't explicitly say that," Ramsey said.

Ramsey said Chau knew the risks, "but it didn't seem to frighten him."

"He believed he was going to heaven, going to be with God if he died," Ramsey said.

For that reason, Chau didn't want anyone to come with him to the island and put their lives in danger, Staver said.

"He went back himself knowing that he would be in danger."

Norwegian geneticist Erika Hagelberg described the larger group of Andaman Islanders as "arguably the most enigmatic people on our planet." They are made up of several tribal groups, who were largely isolated until the island chain was turned into a British penal colony in the 19th century.

Survival International, a nongovernmental group that says it is dedicated to tribal peoples' rights, said Indian authorities should ensure outsiders not make contact with the tribe, because of the risk of disease or threats to their land.

"The Sentinelese have shown again and again that they want to be left alone, and their wishes should be respected," the group said. "The British colonial occupation of the Andaman Islands decimated the tribes living there, wiping out thousands of tribespeople, and only a fraction of the original population now survive. So, the Sentinelese fear of outsiders is very understandable."

'This faith is worth dying for'

In a post on his Instagram page, Chau's relatives said: "He loved God, life, helping those in need, and had nothing but love for the Sentinelese people."

They remembered him as a beloved family member. To others, he was "a Christian missionary, a wilderness EMT, an international soccer coach, and a mountaineer," his family wrote.

To Ramsey, Chau is a modern-day Jim Elliot -- an American evangelical Christian who was killed in a mission in Ecuador in the 1950s. His friend may be able to reach more people in death than in life, Ramsey said.

"This can make a statement to the world that this faith is worth dying for, I suppose," Ramsey said.