Note: All emphasis in the quotes from Ellen G. White Writings (both the underlining and the occasional bolded text), has been added by me. All emphasis in quotes from the Bible are also mine.
Also, unless otherwise stated, Bible quotations are from the NASB with all emphasis added)
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Deception in The Church
Judge By The Right Standard - The Bible - Not Tradition, Creeds or so-called Orthodoxy
Deception in The Church
The word 'deceive' means to cause someone to believe what is not true, regardless of whether the person doing the misleading is himself deluded.
One of the four most dangerous mistakes any Christian can make is to believe that they themselves, their denomination/pastor /spiritual leader, cannot be fooled. If false teachings were easy to spot, it is hardly likely that the Scriptures would warn against deception so very many times.
The 23rd chapter of the book of Jeremiah points out how widespread and treacherous deception can be. He spoke of the false shepherds that were destroying and scattering the sheep, and the "polluted" prophets and priests who spoke visions that came not from the mouth of the Lord, but from their own imaginations, thus leading the people into "futility".
The vast majority of the New Testament books have explicit and often forceful warnings about false teachers. In fact, the warnings against being deceived is a major themes in the New Testament with the words deceit, deceitful, deceitfully, deceitfulness, and deceive, used over and over again.
Paul said that "some" will fall away from the faith. However Jesus was far more explicit. He not only said that many would be led astray and few would find the narrow road, but that even the 'very elect' could be deceived.
Enter through the narrow gate. For wide is the gate and broad is the road that leads to destruction, and many enter through it. But small is the gate and narrow the road that leads to life, and only a few find it. (Matthew 7:13-14).
For false Christs and false prophets will arise and will show great signs and wonders, so as to mislead, if possible, even the elect. (Matthew 24:24 NASB)
Both Peter and Paul very emphatically stated that wolves would arise from the flock itself
"I know that after my departure savage wolves will come in among you, not sparing the flock; and from among your own selves men will arise, speaking perverse things, to draw away the disciples after them. Therefore be on the alert, remembering that night and day for a period of three years I did not cease to admonish each one with tears. (Acts 20:29-31 NASB)
But false prophets also arose among the people, just as there will also be false teachers among you, who will secretly introduce destructive heresies, even denying the Master who bought them, bringing swift destruction upon themselves. (2 Peter 2:1 NASB)
Yet, most theologians, Bible scholars, pastors, etc. sincerely believe they themselves or their denomination, are the sole reliable interpreters of Biblical truths, and will defend to the death their version of what the Bible says. For example, although most people cannot wrap their heads around, much less explain the concept of the trinity, the vast majority of Christians believe it to be an unassailable and inviolable doctrine that comes straight from the pages of Scripture itself. In fact, numerous churches claim that one cannot be saved or called a Christian without believing in the Trinity. But is either claim true?
Read the origins of the trinity for yourself. See part five of THIS article, although reading all the chapters is even more enlightening. Note: Challenging the doctrine of the trinity does not mean challenging the Divinity of Christ and it does not mean challenging the Divinity of the Holy Spirit.
If we are smart, we had better not hang our hats on the peg labeled 'official church doctrine', on what supposedly orthodox Christians/churches think, believe, or teach, nor on what the so called early "church fathers" wrote. And we certainly had better not completely depend on what the learned scholars tell us, however highly respected and/or well known they might be. The question we need to ask is what the Bible teaches, because the only standard by which we can judge the doctrine, beliefs, and traditions of any and all who name the name of Christ is the Bible.
Unfortunately, even if the numerous warning against deception are brought to their notice or they are already aware of them, most of the Christian world is quite convinced that these cautions cannot possibly apply to them. Every Christian seems to wholeheartedly believes that the path they are on is the right one, the beliefs they hold are Biblical, and the practices they indulge in are supported by Scripture.
Christians often play a deadly game of follow the leader, often refusing to accept that well-known and well-respected leaders/preachers/authors can possibly be wrong. All too many are influenced by a charismatic preacher using out of context verses to 'prove' his or her point, others by someone claiming to have heard directly from the Lord.
And, once having decided what to believe, they hang on to it with a deadly grip. However, as the author of Proverbs said.. "There is a way which seems right to a man, But its end is the way of death". (Proverbs 14:12 NASB). See
Why Christians are Deceived.
The Four Most Dangerous Mistakes Any Christian Can Make
Fraternizing With The Enemy
However, what is crucially important is that we...
Judge By The Right Standard, i.e. The Bible, Not Tradition, Creeds or so-called Orthodoxy
Because they hold a number of beliefs that differ from most other Protestants, many Christians, even Christian scholars, are at odds as to whether the Seventh Day Adventist Church is a cult or a Christian denomination - The word usually applied to religious groups that claim to be Christian, but follow the personal interpretation of one or more individuals, rather than the understanding of the Bible accepted by mainstream Christianity. In other words, most Christians arrive at their conclusions based on whether or not these groups are "orthodox"... the word defined by the Oxford dictionary as (Emphasis Added)
"Following or conforming to the traditional or generally accepted rules or beliefs of a religion, philosophy, or practice."
The word "heresy' usually gets the same treatment. For example, Greg Koukl says a heresy is
"teaching that is inconsistent with the creeds and classical teaching of the church."  This leads me to presume that, in spite of Paul warning that the wolves would arise from among the flock, Greg Koukl does not believe that "the church" can be deceived.
Before classifying any group as a cult, Christians need to pause a moment and read the definition of the word (from a theological standpoint). All Emphasis Added
The term 'cult of Christianity' is used of a group, church or organization whose central teachings and/or practices are claimed to be biblical or representative of biblical Christianity, but which are in fact unbiblical and not Christian in nature. 
A cult of Christianity is a group of people, which claiming to be Christian, embraces a particular doctrine system taught by an individual leader, group of leaders, or organization, which (system) denies (either explicitly or implicitly) one or more of the central doctrines of the Christian Faith as taught in the sixty-six books of the Bible. 
Bearing those definitions in mind, it is a tragic fact that most of the modern church can be classified as a 'cult' because two thirds of the teachings (even in so-called orthodox churches) stem from sources, other than the Bible. What is considered 'mainstream' Christianity has gone so far off the rails that it explicitly or implicitly contradicts many of the teachings of the sixty-six books of the Bible.
Thus the object of this article is not to prove whether Seventh Day Adventism is or isn't a cult, but that fundamental SDA doctrine does not conform to what the Bible teaches.
CHAPTER I... History and Beliefs
The Seventh Day Adventist Church - Mission and Distinguishing Doctrines
The Prophetic Ministry of Ellen White
CHAPTER II... Investigative Judgment - Part 1
A 'Spiritual" Fulfillment of Daniel's Prophecy?
CHAPTER III... Investigative Judgment - Part 2
The Cleansing Of The Old Testament Temple
What Was The Sanctuary Defiled by?
Daniel's Vision... What Was He Talking About?
Cleansing The Heavenly Sanctuary
The Overshadowing Theme of Hebrews
Summary and Conclusion
When Does the Atonement Actually Happen or, to put it another way... When Are We Finally Saved?
CHAPTER IV... The Doctrine of The Shut Door
The Metamorphosis of The Shut Door Doctrine
Ellen White's Shut Door Visions
An Extremely Tenuous Defense of the Metamorphosis of The Shut Door Doctrine
So When Does The Door Actually Shut?
Chapter V ... Was Ellen White a True Prophetess of God?
Ellen White: Spiritual Leader and 'Prophet'
Life, Career and Position in the SDA Church
The Biblical Tests For a Genuine Prophet of God
The Orchard Test?
Dreams and Visions ... Mark of a True Prophet of God?
Fulfilled Predictions ... Mark of a True Prophet of God?
Ellen White's False Prophecies
Chapter VI... Did Ellen White’s Teachings Harmonize With God’s Word?
Harmony With God's Word?
Ellen White's Out of Context and Partially Quoted Verses, Contradictions and
Additions to The Word of God
Five Biblical Facts Ellen White Got Wrong
(The modern church doesn't get many of them right either)
CHAPTER VII... Ellen White Health Reform
Biblical Dietary Restrictions
Ellen White's Health Reform Included Some Very Bizarre Statements
In any case Seventh-day Adventists Cannot Have It Both Ways
CHAPTER VIII .... Ellen White's Other Teachings
Ellen White's Teachings on Phrenology, Photography, Furnishings, Jewelry, Labour Unions
Her Preposterous Opinion on Leisure Activities like Sports, Chess and Checkers, Singing, Concerts and Reading
Vitriolic Criticism of Others
Examples of Plagiarism
CHAPTER IX ... Ellen White's Visions
Had little or no importance in the general scheme of things. Much to the contrary, they ranged from the childish
and farfetched to the petty and inconsequential. Many simply served to bolster the Adventist message.
What Was The Source of Ellen White's Visions?
The physical phenomena that occurred, and continue to occur, during Shamanistic visions bear a remarkable similarity to what occurred during Ellen Whites visions. Also to be noted is the fact that spirits often begin to influence a Shaman's life after a life crisis such as near-death, accident or serious illness. In other words, they take advantage of people when they are at their most vulnerable.
Ellen White's 'Guides' and Advice From a Dead Husband
CHAPTER X ... The Sabbath
Is the Sabbath a key issue in Christianity, and will those who do not keep the Sabbath eventually receive the 'mark of the beast'?
CHAPTER XI ... The Clear Word Bible
is widely used within the Seventh-day Adventist churches.. However, the CWB is a paraphrase that includes the teachings of Ellen White, with many words and phrases altered to reflect those teachings.
 Greg Koukl. What’s the Difference Between Bad Theology and Heresy?
 Apologetics Index. Cult of Christianity. http://www.apologeticsindex.org/2765-cult-of-christianity
 Alan W. Gomes. Unmasking the Cults Paperback – Zondervan Guide to Cults and Religious Movements. Zondervan (May 11, 1995)