As I urged you when I went into Macedonia, stay there in Ephesus so that you may command certain people not to teach false doctrines any longer or to devote themselves to myths and endless genealogies. Such things promote controversial speculations rather than advancing God’s work—which is by faith. The goal of this command is love, which comes from a pure heart and a good conscience and a sincere faith.
Some have departed from these and have turned to meaningless talk. They want to be teachers of the law, but they do not know what they are talking about or what they so confidently affirm.
~1 Tim 1
What is Paul warning of there? What is the false teaching being spoken of? It's not exactly clear, but we can read that the teachings described are not coming from a pure heart of love and a sincere faith, so maybe that helps.
Also, there is the reference to those who "promote controversial speculations" instead of "advancing God's work."
If anyone teaches otherwise and does not agree to the sound instruction of our Lord Jesus Christ and to godly teaching, they are conceited and understand nothing. They have an unhealthy interest in controversies and quarrels about words that result in envy, strife, malicious talk, evil suspicions and constant friction between people of corrupt mind, who have been robbed of the truth and who think that godliness is a means to financial gain.
~1 Tim 6
False teachers, Paul tells us, are those who do not agree with the sound teaching of Jesus and "godly" teaching (left undefined, but we probably generally share a good notion of at least some godly teachings, things like the fruit of the Spirit, perhaps?)
They also have an unhealthy interest in controversies and quarrels that lead to malicious talk and evil suspicions, for what it's worth.
Remember Jesus Christ, raised from the dead, descended from David. This is my gospel...
If we died with him,
we will also live with him;
if we endure,
we will also reign with him.
If we disown him,
he will also disown us;
if we are faithless,
he remains faithful,
for he cannot disown himself
Keep reminding God’s people of these things. Warn them before God against quarreling about words; it is of no value, and only ruins those who listen. Do your best to present yourself to God as one approved, a worker who does not need to be ashamed and who correctly handles the word of truth. Avoid godless chatter, because those who indulge in it will become more and more ungodly. Their teaching will spread like gangrene. Among them are Hymenaeus and Philetus, who have departed from the truth. They say that the resurrection has already taken place, and they destroy the faith of some.
~2 Tim 2
Here Paul lists avoiding godless chatter, quarreling about words, and not believing in Jesus' death and resurrection.
They will secretly introduce destructive heresies, even denying the sovereign Lord who bought them—bringing swift destruction on themselves. Many will follow their depraved conduct and will bring the way of truth into disrepute. In their greed these teachers will exploit you with fabricated stories. Their condemnation has long been hanging over them, and their destruction has not been sleeping.
Bold and arrogant, they are not afraid to heap abuse on celestial beings; yet even angels, although they are stronger and more powerful, do not heap abuse on such beings when bringing judgment on them from the Lord. But these people blaspheme in matters they do not understand...
Their idea of pleasure is to carouse in broad daylight. They are blots and blemishes, reveling in their pleasures while they feast with you.With eyes full of adultery, they never stop sinning; they seduce the unstable; they are experts in greed—an accursed brood! They have left the straight way and wandered off to follow the way of Balaam son of Bezer, who loved the wages of wickedness...
For they mouth empty, boastful words and, by appealing to the lustful desires of the flesh, they entice people who are just escaping from those who live in error. They promise them freedom, while they themselves are slaves of depravity—for “people are slaves to whatever has mastered them.”
~2 Peter 2
In that passage, we find a goodly numberr of more specific traits of false teachers.
1. "carousing/eyes full of adultery..." some sort of sexual sin, it would appear
2. "Experts in Greed" - those who chase after money
3. "Denying the sovereign Lord who bought them," denying Jesus as the son of God, perhaps?
4. "Not afraid to heap abuse on celestial beings..." Bold and arrogant
5. In Coffman's commentary, he notes the "fabricated stories" or "feigned words" indicates those who'd deliberately teach false doctrine, not the merely deluded.
More from Coffman...
This includes reviling "magistrates," (celestial beings) as Macknight said, but much more is meant. It is a loudmouthed, blasphemous declamation against all that is high, honorable, or holy. Authority of any or all kinds is anathema to this class.
For there are many rebellious people, full of meaningless talk and deception, especially those of the circumcision group. They must be silenced, because they are disrupting whole households by teaching things they ought not to teach—and that for the sake of dishonest gain.
One of Crete’s own prophets has said it: “Cretans are always liars, evil brutes, lazy gluttons.” This saying is true. Therefore rebuke them sharply, so that they will be sound in the faith and will pay no attention to Jewish myths or to the merely human commands of those who reject the truth...
They claim to know God, but by their actions they deny him. They are detestable, disobedient and unfit for doing anything good.
Again, Greed is a telling characteristic of false teachers. Also "meaningless talk" and deception. We also have those here, it appears to me, who are adding to grace, by demanding IN ADDITION to being saved by grace, we also have to be circumcised.
Additionally, we have the suggestion that false teachers are liars (indicating deliberate deception - and with apologies to any decent Cretans out there...) who embrace "jewish myths" and "mere HUMAN commands."
“Beware of the false prophets, who come to you in sheep’s clothing, but inwardly are ravenous wolves. “You will know them by their fruits.
That's Jesus weighing in on the topic, stating clearly that we will know them by their fruits, by their ill-behavior.
Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world. This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, but every spirit that does not acknowledge Jesus is not from God...
Dear friends, let us love one another, for love comes from God. Everyone who loves has been born of God and knows God. Whoever does not love does not know God, because God is love...
God is love. Whoever lives in love lives in God, and God in them...
Whoever claims to love God yet hates a brother or sister is a liar.
~1 John 4
John stating the familiar theme of knowing false prophets by their denying that Jesus was the son of God (probably referring to gnostics, who denied Jesus had an actual earthly body) and the evidence of love, for Love is from God and "EVERYONE who loves has been born of God," quite the astounding statement.
For if someone comes to you and preaches a Jesus other than the Jesus we preached, or if you receive a different spirit from the Spirit you received, or a different gospel from the one you accepted, you put up with it easily enough.
For such men are false apostles, deceitful workers, disguising themselves as apostles of Christ. And no wonder, for even Satan disguises himself as an angel of light. Therefore it is not surprising if his servants also disguise themselves as servants of righteousness...
~2 Cor 11
It appears to me that false teachers can be recognized
1. By their actions - by lives that are not full of grace, love, purity, self-control, kindness, gentleness, who slander and gossip and are greedy and who sexually act out.
2. by their being deliberate in their false doctrine, not speaking of those who are merely mistaken in sincerity, but blatantly lying.
3. By denying Jesus was sent of God.
4. By teachings that deny Jesus' teachings.
5. By teachings that deny Jesus' literal humanity.
For your consideration...
Jesus knew their thoughts and said to them, “Every kingdom divided against itself will be ruined, and every city or household divided against itself will not stand. If Satan drives out Satan, he is divided against himself. How then can his kingdom stand? And if I drive out demons by Beelzebul, by whom do your people drive them out? So then, they will be your judges. But if it is by the Spirit of God that I drive out demons, then the kingdom of God has come upon you.
“Or again, how can anyone enter a strong man’s house and carry off his possessions unless he first ties up the strong man? Then he can plunder his house.
“Whoever is not with me is against me, and whoever does not gather with me scatters. And so I tell you, every kind of sin and slander can be forgiven, but blasphemy against the Spirit will not be forgiven. Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come.
“Make a tree good and its fruit will be good, or make a tree bad and its fruit will be bad, for a tree is recognized by its fruit. You brood of vipers, how can you who are evil say anything good?
So, here again, we see the measure of "good fruit," of those who live good lives, as being evidence AGAINST the "false teacher" charge. And it comes with the warning: "YOU BROOD OF VIPERS, how can you who are evil say anything good..." and "Anyone who speaks against the Holy Spirit will not be forgiven."
owch. So, I would say that, while it is true it is very wrong to be a false teacher - one according to the Bible who deliberately teaches wrong teachings - but it is equally true, it seems to me, to be VERY wrong to falsely accuse someone of God to be a false teacher.
It seems that those who are false teachers can be forgiven and brought back into the family of God, but those who falsely ACCUSE of false teaching, it sounds like they can't be!