This is/was a very bad cult which managed to destroy many lives. [It has re-emerged in a new form under another name, something like new church of God*]. It is extremely legalistic and there is much on the web about them, posted by ex members who provide quite an expose’ of this group. It is difficult for ex members of this group to obtain release from the indoctrination that amounts, as always, to brain washing. Sometimes a lot of psychological help has to be obtained to gain relief from the aftermath of this, and other cults.
The problems with these groups always seem to result from the wrong view of atonement, although I am not sure of the specifics of this one, or the Mormons for that matter. But whenever the wrong view of atonement is taught, there emerges all kind of legalisms – That is, that the freedom the gospel delivers is hidden behind legalistic requirements which only serve to maintain religious slavery which is similar to Judaistic principles. Whereas the gospel preaches release from law, these groups create their own laws [they are without excuse] as well as twisting the existing scriptures to suit themselves. Often there is an emphasis on prophesy [futuristic] as a way of postulating their supposed authority.
Where the correct view of the atonement appears to be present (seems unlikely) then other reasons must be sought for it. The thing about having the correct view of course, is that it results in the freedom which the gospel was meant to bring, so if freedom is replaced by legalisms, you can immediately be very suspicious about what is going on. The freedom that the Spirit brings is unmistakable and normally excludes the possibility of such errors occurring.
This is a link or links to other web pages which may help to reveal the character of these groups. http://ironwolf.dangerousgames.com/exwcg/
“The painful Truth” https://hwarmstrong.com/grown.htm
*[Possibly “Grace Communion International”?]
” Recovering from Armstrongism” http://www.ukapologetics.net/08/ARM/1/index.htm