The books feature the formation of a "Tribulation Force" to resist the Antichrist. There is nothing like this even hinted at in the Bible.

 Prayers to the dead is taught in one of the books. During the funeral for a Tribulation-era pastor, one of the leaders of the Tribulation Force is depicted as praying to the dead man (vol. 3, Nicolae, p. 315).

 New Age/occult out-of-body experiences are encouraged. One of the characters in The Indwelling has an elaborate and richly detailed out-of-body experience (OBE); LaHaye glorifies OBE as a good way to pray. LaHaye's OBE account is identical with OBEs described and marketed by countless New Agers on the Internet and in myriads of occult based books (vol. 7, The Indwelling, pp. 232-248).

 One of the very questionable doctrines the Left Behind series teaches is that at the time of the rapture pregnant women will suddenly become un-pregnant (that is, the unborn babies will be taken in the rapture and will leave the unsaved mother, who is left behind, many pounds lighter!). A rapture for embryos! (vol. 1, Left Behind, pp. 46-47).

 The Christians in the books make major decisions based on their intuitions and human reasoning rather than on Bible precepts. For example, two of the heroes of the Left Behind series become romantically involved. Though they discuss the matter and wonder if it is God’s will, they get married and have a child in the midst of the Great Tribulation, with no apparent consultation of the Scriptures. First Corinthians 7:29-35 would have settled their question. Another example of this is the decision made by two of the members of the "Tribulation Force" to work for the Antichrist, one as his pilot and one as the head of a news magazine. They question the propriety of their decision, but instead of consulting the Scriptures, they "feel" it is God’s will. One would think that 2 Corinthians 6:14-18 and Revelation 18:4 would easily have settled that question.

 Female Bible teachers are acceptable. Book three has two female members of the "Tribulation Force" trained to teach the Bible to mixed groups of people (vol. 3, Nicolae, pp. 354,355). Contrast this teaching with 1 Timothy 2:12.

 Books 8 & 9 teach that recipients of the mark of the beast can still be saved. In The Mark, "the Chang scenario" is developed, whereby a character receives both the mark of the beast and the sealing of the Lord. In Desecration, Chang's dual-marking was justified in the storyline. The question about how a Christian can have the mark of the beast and still be saved has been asked many times by perplexed readers on the Left Behind messageboard, and was answered on the FAQ page at LeftBehind.com. "God looks on the heart," says Jenkins. "He [Chang] was a believer first, and thus, always." Notice that Jenkins did not say, "It is only fiction, so do not take it seriously." He also did not say, "You must understand that in real life, no Christian may receive the mark of the beast and be saved" (cf. Matt. 10:32,33). [LaHaye and Jenkins also teach this "true Christians may deny the Lord Jesus Christ before men and still inherit eternal life" concept in their non-fiction book Perhaps Today -- see LaHaye report for more details.]

 The pope is raptured away! The books depict the Roman Catholic pope as a saved man who is taken away in the Rapture. This is perhaps the most serious and dangerous error in all the books. It encourages the ecumenical movement and its powerful back-to-Rome emphasis. It adds support to the false ecumenical concept of "evangelical Catholicism." Of course, for any pope to be saved, he would have to reject what his own church teaches. The Roman Catholic Church dogmatically teaches that works and sacraments are necessary to salvation, and that man cannot be saved by the grace of Christ alone. So with this series, LaHaye and Jenkins help Chuck Colson, Billy Graham, Charles Stanley and the rest of the Catholic sympathizers to further blur the clear line of demarcation between true Christianity and apostate Christianity.

 * This report (and the Notes below) was excerpted and/or adapted from the following sources: (1) "Left Behind: Tolerable Fiction, Intolerable Theology," 6/28/01, FBIS; (2) "Left Behind: One of Satan’s Latest Wiles," Raymond Blanton, The Perilous Times, Feb/Mar 2001; and (3) The "Left Behind" official Internet web site.

There are blatant New Age and occultic markings on the Left Behind Series (including Tsion referring to his  Bible as a talisman!)

 Newborn Christians are depicted in the novels as putting on guile and hypocrisies so that they can supposedly serve God.  The storyline presented the sins of guile and hypocrisy as the epitome of cleverness, and as valuable tools to aid Christians in overcoming Antichrist.

The example that Christians can sin in order to serve God is not exalting Jesus Christ.

 If You Are Supernaturally Protected From Harm, Then You Should Assume You are Right With God? (The Remnant p. 14)

Murder For "The Cause" The Remnant p. 7

"This is war":  Authors' Justification For Christians to Murder The Indwelling p. 49,90 Assassins p 370, 372, 382, 393, 396, 410

The "Elena Kill" The Remnant p. 167, 168

Christian characters using terrorist talk to describe George's murder of Elena The Remnant p. 175

Mac wanted to "take a few out of them" as well Remnant p. 196

Authors Did Not Portray George Repenting of Murder; Instead, This Sin Was Justified The Remnant p. 167, 168

Christians Being Made to Look Like Terrorists If the world did not know it before, they know it now: Christians want to kill and will kill in serving "the cause." 

The Chinese government calls what they define as terrorist groups by the name of "forces." One is a force, two or more are forces. What is more, they are presented of course, as evil forces. These separatist, extremist forces fight in what they believe is a holy war. 

False I.D.'s a Way of Life For Terrorists Assassins, p 189

The Tribulation Force Dubbed The "Force" The Remnant p 268 (Occult reference: Satan is also called The Force) Left Behind p 420

Serving "a Cause" Totally Different From Serving the Person of the Lord Jesus Christ

Interchanged God and Satan Assassins p 165, 170, 171
Leon (the Satanist) refers to “God” not “god”

The reader is given the examples of the Christian losing all hope, and the satanist praying to God. This is perverted and glorifies Satan's follower as having faith (in God) and the Christian losing hope.  The reader sees the Most High God depicted as answering the satanist's prayer by having a Christian shot twice while the satanist remained protected.  God was depicted as protecting Satan's follower, and not protecting His own follower.

http://www.libertytothecaptives.net/satan_God_interchanged.html

New Age Imagery Helps Popularize Prophecy, http://www.crossroad.to/articles2/prophecy.htm, Berit Kjos

This is Truth With a Capital T?

The example that a Christian may "pretend" to worship the beast is "Truth with a capital T"?

The example that a Christian may have the mark of the beast forced on his person (because he refused to confess Jesus Christ before men) and remain saved is "Truth with a capital T"?

The teaching that if a Christian has the mark of the beast he is still saved because "God looks on the heart," relates Jerry B. Jenkins. "He was a believer first, and thus, always" is "Truth with a capital T"? (See FAQ)

The teaching (via a sermon!) that Jesus' death on the cross was a martyrdom is "Truth with a capital T?"

The example that an unmarried Christian is ashamed of being a virgin is "Truth with a capital T?"

The teaching that if you do not disappear before the tribulation begins you can be certain you are not a real Christian is "Truth with a capital T"?

The teaching that the Bible teaches that real Christians will have visible cross-shaped marks on their foreheads during the tribulation is "Truth with a capital T? 

The teaching that God will lead his people to work for Antichrist and use Satan's tools of sin is "Truth with a capital T"?

Christians behaving like terrorists is "Truth with a capital T"?

The example of Christians acknowledging Antichrist as lord is "Truth with a capital T"?

The example that a Christian may practice Transcendental meditation to gain a sense of peace under duress is "Truth with a capital T"?

The example that a Christian can confess Antichrist as the risen lord in order to serve God is "Truth with a capital T"?

The example that there can be a special category of Christian during the tribulation: one that has the seal of God and the mark of the beast is "Truth with a capital T"?

The example that God will lead a Christian to deny Jesus Christ in order to save his own life (and not repent of this sin) is "Truth with a capital T"?

The teaching that abortion might be justifiable in the case of rape or incest is "Truth with a capital T"?

If you have read Nicolae and did not realize that Rayford communicated to Hattie that a rape or incest victim might be justified in having an abortion then you have been manipulated.

"Shirking responsibility" is the euphemism for abortion in the context of this story.
(Rayford is speaking to Hattie) "I'll even buy the argument that perhaps you regret the idea of having a child at all and would not be the best mother for it. I don't think you can shirk responsibility for it the way a rape or incest victim might be justified in doing." (Nicolae p. 296)

The "right to choose"
"But even in those cases, the solution isn't to kill the innocent party, is it? Something is wrong, really wrong, and so people defend their right to choose." (Nicolae p. 296)

"What they choose, of course, is not just the end of a pregnancy, not just an abortion, it's the death of a person."  But which person? One of the people who made a mistake? One of the people who committed a rape or incest? Or one of the people who got pregnant out of wedlock? No, the solution is always to kill the most innocent party of all." (Nicolae p. 296)

you will see twice they used the word solution along with the word kill

Rayford had gone too far, and he had known it. He had glanced up at Hattie holding her hands over her ears, tears streaming down her face. He had touched her arm, and she had wrenched away. (Nicolae p. 296-97)

(If a Christian tells a pregnant woman seriously considering abortion that she would be killing a person he is going too far?)

Rayford had been angry with himself. Why couldn't he learn? How could he sit there spouting all that? (Nicolae p. 297)

Earnestly trying to get a woman to see that abortion means the death of a person is "spouting all that"? Rayford is angry with himself for forthrightly telling Hattie that abortion means death?  Do you see the implication?

He believed it, and he was convinced it was God's view. It made sense to him. But he also knew she could reject it out of hand simply because he was a man. (Nicolae p. 297)

Rayford was convinced it was God's view? Since when does God have a view or opinion? The authors are made God's word appear relative when God's word is absolute!  God has given us his Word on the matter of murder.  His word is not an opinion or merely a way of looking at things.

"I want you to forgive me for anything I said that hurt you personally or insulted you. I hope you know me well enough by now to know that I would not do that intentionally." (Nicolae p. 297-98)

 Rayford said nothing insulting, and he was not unkind.  There is the implication that standing firm on abortion and explaining clearly that it is the killing of a child could be insulting.

Rayford was angry with himself. (Nicolae p. 298)

He may not have handled it the best way, but he knew trying to fix it now would accomplish nothing. (Nicolae p. 299)

Sinning is Presented as an Opportunity to Serve God!

Satan is an expert at helping believers be pragmatic--the end justifies the means.

What?? The authors are using the character's dialogue and thinking to influence the reader to think that God put the compromising words into Buck's mouth!

When Buck Cameron met with the Nicolae Carpathia (Antichrist) he was presented with an opportunity to honor the Lord Jesus Christ...to witness...to confess Christ before men. He knew very well the person to whom he was speaking was an enemy to God, and he didn't know what to say when Nicolae asked him this question: "What motivates you?"

"Buck prayed quickly and silently. God, Christ, salvation, the Tribulation, love, friends, lost souls, the Bible, learning, preparing for the Glorious Appearing, New Hope Village Church, Chloe. These were the things that motivated him, but should he say that? Should he? God, give me the words!"

"I am motivated by truth and justice," Buck said flatly." Understandably, Nicolae did not perceive this as any kind of assertion of loyalty to the Lord Jesus Christ, and the conversation went on unhindered by Buck's unashamed confession of Christ, because he did not give it.

 Excerpts compiled from http://www.libertytothecaptives.net. 
Please check there for additional information.