The Christian Patriots Defense League that Mattern was part of after leaving the Klan was white supremacist, anti Semitic, survivalist, with a persecution complex claiming the gov't and society are out to destroy Christians. They claimed white Americans were the chosen people, not Jews. They were also a paramilitary militia, with gun clubs and training for an end times they imagine will come soon.
So the timeline is early mid 60s he was a Marine. After leaving, he's with United Klans for 14 years. Then he's with CPDL for perhaps 3-6 years.
At that point he begins his impersonation. There's a lot of obvious parallels with Asa Carter, the KKK leader who became Forrest Carter and posed as a Cherokee author, lying that his ancestors were on the Trail of Tears.
Both cases also show that journalists and publishers can be very gullible, not do any research when someone panders to what they wish was true, and they have no actual NDNs around. Claiming to be a priest of the "American Indian Church"? Claiming that this paler version of Willie Nelson with vaguer hippie talk was a fullblood Lakota medicine man?
The last thing both men do is show the thin line between white supremacy and culture theft. I've seen fans of Carter's book claim he had some kind of change of heart. I'm sure Mattern's cult and his "adopted sons" will claim the same. But no, claiming that whites have the "right" to steal Native identity, beliefs, and practices is just as white supremacist as KKK beliefs. It's just a more elaborate spiritual colonialism, a way for whites to assuage their guilt over their ancestors stealing Native land.
The Hawkwind Earth Renewal Cooperative that his cult began as part of has ties to some other frauds. "Mary Thunder" endorses them as allies. Also Charla Thomas/Tarwater of the Bear Tribe cult. She co founded HERC along with "Reverend" Charla Herman, who now run an alchemy academy.