The Greek word apostasia (apo, from, and stasis, station, standing, or position) originally meant desertion of a post or the giving up of a state of life. But in the early church apostasy meant the voluntary, complete abandonment of Christianity by either going back to Roman paganism, or Judaism, or even giving up religious beliefs altogether. The first few centuries of Christianity believed it to be so grievous a sin that the church would excommunicate the apostate (as if the apostate would care?) and they would leave them to the mercy of God.
After the Decian persecution of the mid third century, so many Christians committed apostasy and later repented that the church at the Synod of Carthage relaxed the penalties and forgiveness was granted more easily. (1 Theodosian Code XVI title 7, De Apostatis; title 8, De Judaeis, “Corpus juris romani ante-Justinian (Bonne, 1840) 1521-1607; Code of Justinian I, title 7, De apostatis I. c. 60, 61.)
After Christianity became the state religion, the emperor withdrew all civil rights of apostates. To entice anyone into apostasy brought the death penalty.
The word appears only twice in scripture:
Acts 21:21 This is where Judaisers entreated Christians to be circumcised and return to Judaism under the Old Covenant. This was not a form of Christianity but a refutation of it.
2 Thess. 2:3 “Let no one in any way deceive you, for it will not come unless the apostasy comes first, and the man of lawlessness is revealed, the son of destruction.”
As an Adventist or former Adventist you will immediately understand that this text was used by Ellen White to teach that the last day apostasy is by the Protestant church following the influence of Catholicism. (See Great Controversy, pp 445-447.)
White’s view was that you could remain a Christian in 9 out of 10 commandments but that it was the breaking of the Sabbath commandment that defined you as apostate. This is a misunderstanding and misapplication of the word as understood during Bible times as well as a millennia and a half after Christ.
Apostasy is not a serious backsliding or heresy. Apostasy is turning your back on Christianity and defining yourself as an atheist, Buddhist, Moslem, Jew, etc. You openly deny Christ and publicly call yourself something else. You no longer attend a Christian church.
Here are some EGW texts that show her misapplication of Paul’s pronouncement of “apostasy.”
For many centuries thereafter an apostate form of Christianity held sway. The truth was suppressed and lost sight of, and ignorance prevailed. ...These centuries of apostasy are correctly designated in history the “Dark Ages.” CET 241-242
The apostle Paul, in his second letter to the Thessalonians, foretold the great apostasy which would result in the establishment of the papal power. ... Even at that early date he saw, creeping into the church, errors that would prepare the way for the development of the papacy... GC 49-50 See also GC 443-444
When the leading churches of the United States, uniting upon such points of doctrine as are held by them in common, shall influence the state to enforce their decrees and to sustain their institutions, then Protestant America will have formed an image of the Roman hierarchy, and the infliction of civil penalties upon dissenters will inevitably result. GC 445
Ellen White’s prediction misunderstood what Paul was saying. Paul foretold that Christians would become unbelievers, atheists or perhaps join another religion altogether. This great apostasy would not be those who claimed to be His Church, call themselves Christians and worship Him and simply not fully understand or even be deceived. According to Ellen, all Christians for a thousand years would be then apostates. This is not logically sound but anachronistic thinking. Ellen misunderstood the word heretic for apostate; heresy for apostasy.
Paul wants us, as Christians, to be looking for a great falling away of Christians who choose to believe evolution or join the Muslims or become Buddhist. Apostates are not bad Christians, they are not Christians at all and publicly denounce their Christianity. There will of course be lots of heretics... those who claim to be Christians and do not follow Christ’s New Covenant Commandments and teach heresies, but this is a different group. Apostasy is the step after being a stubborn, unrepentant heretic, but it doesn’t have to ever go that far.
So Adventist calling other Christians apostate is unsound exegesis of Paul’s words as well as being rather uncharitable to fellow Christians.