Mr. Armstrong stressed that the ministry use extreme caution when inviting new prospectives with strong Pentecostal, Jehovah's Witness or Seventh Day Adventist background. Mr. Armstrong felt that these people very seldom come out of their former religion. (Emphasis mine.)Can't let the ex-Jehovah's Witness or ex-Seventh Day Adventist see that so much of Armstrongism is pilfered from them! Or let them tell the members!
Both movements, it must be noted, are descended from the Millerites of the 1830s/1840s, just like Armstrongism. This is one reason why these religions bear some similarities to Armstrongism. The other reason is that HWA plagiarized (stole) some of his doctrines from them.
If the (Armstrongite) 'truth' is so powerful why did 'God's End Time Apostle' fear these people so much? Because they possess evidence that HWA just stole other ideas and they could see that and possibly tell others about it. They are an ever present reminder that HWA was not special at all in his alleged 'revelations', and this no doubt was the cause of this fear.