Robin Brace, a former Worldwide Church of God member and a former minister, has written extensively on the UK site Outreach Trust which works to help people recover from abusive churches, legalism, and defending the faith.
This is some of the information he has written about William Miller and the erroneous church movements he fathered (including Adventism, Jehovah's Witnesses, Armstrongism, and even Mormonism).
Note the highlighted section below. Today's Church of God's self-appointed leaders do this exact same thing. They despise the cross and the atoning work and place their entire focus upon fantastical and absurd prophecies interpreted according to their "understanding". This is particularly true of Bob Thiel and his hatred of the cross. The cross and Jesus take the focus off of him.
One cannot stress enough the desire of the early Americans to be free of religious control after having suffered often in the Old World because of its excesses. This hunger for freedom led to a powerful sense of independence and a resulting desire to rediscover Christian community and experience. Much good came from this pursuit of a more personal Christianity, but it also led to an atmosphere in which idiosyncratic beliefs were often tolerated in ways they would not have been in the Old World.
By the 1840s, overlapping historically with the early development of the Mormon religion, William Miller sought to refocus Christianity away from Christ’s atoning work on the cross and instead preached an envisaged, imminent second coming supported by his interpretations of Bible prophecy (especially that found in Daniel and Revelation). He was drawing on strands of sensational teaching which were in no way new. Similar arguments had been attempted before in the Old World but had not prospered because of the wide accessibility of more deeply grounded biblical theology. The New World, however, was determined to be “open” religiously; this American, individualistic freedom assisted the new, exciting adventist worldview and provided an environment in which it could flourish.
All the “adventist” cults and sects—which are American phenomena—can be traced to the legacy of Miller. It matters not whether we speak of Joseph Smith, Ellen White, Hiram Edson, Joseph Bates, Charles Taze Russell (who became the first leader of the Watchtower Society—later, Jehovah’s Witnesses—in 1896), or Herbert W. Armstrong who founded what became the Worldwide Church of God in 1933—these theological mavericks who posed as Christians all reflected America’s individualistic freedom and bore the marks of William Miller’s “gospel”, replacing the finished work of Jesus on the cross with the alarm: “Jesus is coming; get ready!”
Miller’s new approach had proven to be so popular that by 1844, F.S. Mead calculated in his A Handbook of Denominations in the United States, p 20, “… there were between 50,000 and 100,000 adventists in North America.”
Miller’s doomsday legacy continues to evolve; David Koresh, of Waco, Texas fame, was also an adventist, originally of the Seventh-day Adventist sect, later leaving to pursue his own highly idiosyncratic theological path.
Today, Miller is quite famous for his date-setting for Christ’s return based upon his understanding of the apocalyptic books of Daniel and Revelation. Inevitably, however, his dates failed, and many gave up adventism; others went into mental asylums, and some committed suicide. They had given up everything, even leaving their crops unharvested, because they believed Jesus would come and take them away.
Others, however, were not deterred by Miller’s failed prophecies. These adventists refused to accept the fact that they had believed a lie and had discounted the clear teaching of Jesus: “But concerning that day or that hour, no one knows, not even the angels in heaven, nor the Son, but only the Father” (Mk. 13:32). This group never gave up Miller’s approach of refocusing adventists away from the cross and toward the second coming. As time passed, their methods included prophecy, legalism in various forms, and, inevitably, all the pet theories of each of the founders of the sects that emerged from those persistent Millerite faithful.
Miller, like the founders of almost every adventist cult who would follow him, had little deep knowledge of the word of God and had not been a long-term practicing Christian. He had never studied Greek or Hebrew, and it is known that he only used the Bible and Cruden’s Concordance in his work. He was unaccountable and marched to his own theological drum.
In fact, all the subsequent sect-founding adventists followed a similar “me-only” approach, believing that God was revealing new truth—only to them! They even rejected the new understandings of other adventists. In short, their Milleresque methods became notable for their sublime senses of self-sufficiency!
These men and women had never been masters of even one of the biblical languages in which the inspired texts were originally written. Neither were they prepared to check their conclusions against the more time-honored conclusions of men such as Luther, Calvin, Augustine, and others. Consequently, it has been easy for adventists to hold their ground, since they have never felt the need to defend their teachings against the more thoroughly biblically-grounded teachings of the Christian world. In adventism itself, typically, the people are held in subjection to various charismatic leaders and do not dare pose questions. Further, most such sects have painted a picture (also very much part of adventism) that they alone have all truth and that those who hold other biblical views are the tools of Satan! Robin Brace: Moving Away from Legalism