What is the great falling away?
The great falling away is the large scale departure of the professing church from the truth of the Gospel, which Scripture says must happen before the antichrist is revealed and before the Day of the Lord comes.
The great falling away comes straight out of Paul's words to the Thessalonians: "Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition" (2 Thessalonians 2:3-4). This falling away is not the same thing as an ordinary Christian slipping into sin for a season. A backslider is a born again believer who stumbles into sin but is still able to repent and be forgiven. The falling away is far more serious: it is a total abandonment of the faith, a true apostasy. Those who fall away are saints who begin to exalt their own lifestyle, mindset, or philosophy above the Gospel itself, in effect worshiping their own way instead of God. Because of this they are rejected by God and become castaways, or reprobate, a word that literally means the reversal of what was once approved.
This is not a future or hidden thing happening somewhere else. It is described as something already underway in the church right now. Many believers have drifted from sound doctrine, embracing teachings that flatter the flesh, chase itching ears, and turn people toward fables rather than truth (2 Timothy 4:3-4). Doctrines named as part of this drift include Dual Nature, Faith and Prosperity, Dominion Now, and Once Saved Always Saved, all of which are said to point the church toward sin rather than holiness. The result is that the church, as a whole, is described as having actually fallen, not merely wavering, because it fights over lesser things while righteousness itself has become an offense to many within it.
The times are compared to the days of Elijah, when Jezebel silenced or killed the true prophets of God and raised up hundreds of prophets of Baal. In the same way, voices of the world today, in entertainment, media, and politics, have taken the place once held by true prophetic voices, while many genuine believers remain hidden away, hesitant to stand for holiness because of fear of rejection or persecution. The falling away therefore describes both the corruption spreading through the visible church and the quiet withdrawal of the faithful who feel they must hide rather than speak.
Because this falling away must occur before the man of sin, the antichrist, can be revealed, it is treated as a sign that his revelation and the coming of the Lord are drawing near. The occurrence of this widespread departure is presented as proof that the church is living in the very days that Paul described, and that the timeline leading to the antichrist's appearing is already unfolding.
The scriptures
Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition; Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God.
For the time will come when they will not endure sound doctrine; but after their own lusts shall they heap to themselves teachers, having itching ears; And they shall turn away their ears from the truth, and shall be turned unto fables.
Thus saith the LORD, Stand ye in the ways, and see, and ask for the old paths, where is the good way, and walk therein, and ye shall find rest for your souls. But they said, We will not walk therein. Also I set watchmen over you, saying, Hearken to the sound of the trumpet. But they said, We will not hearken.
For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.