God has always been particularly harsh when dealing with false teachers. Everyone who rejects Him will be judged. It is bad when one rejects Him, and it is infinitely worse when one also causes others to reject Him. Prophets that turned the people of Israel away from God were actually to be killed.
Deuteronomy 13:5: And that prophet, or that dreamer of dreams, shall be put to death; because he hath spoken to turn you away from the Lord your God, which brought you out of the land of Egypt, and redeemed you out of the house of bondage, to thrust thee out of the way which the Lord thy God commanded thee to walk in. So shalt thou put the evil away from the midst of thee.
False prophets were not limited to Old Testament times.
Mark 13:22: For false Christs and false prophets shall rise, and shall shew signs and wonders, to seduce, if it were possible, even the elect.
Matthew 18:6: But whoso shall offend one of these little ones which believe in me, it were better for him that a millstone were hanged about his neck, and that he were drowned in the depth of the sea.
If you learn nothing else from this Bible Study, learn this one thing!!! Every cult and every false teacher will deny the Diety (that he is God) of Christ! There will be many clever and complicated ways in which they will try to focus your attention away from this belief of theirs, or to disguise it. Do not let them fool you on this issue. Being in a pulpit or on radio or TV is not a guarantee someone will not be a false teacher.
I John 4:1-3: Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world. Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God: And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is in the world.
The Bible gives very specific instructions on dealing with false leaders. And of course killing them is not an option for Christians.
Matthew 15:14: Let them alone: they be blind leaders of the blind. And if the blind lead the blind, both shall fall into the pit.
Matthew 7:15: Beware of false prophets, which come to you in sheep*s clothing, but inwardly they are ravening wolves.
I Timothy 6:3-5: If any man teach otherwise, and consent not to wholesome words, even the words of our Lord Jesus Christ, and to the doctrine which is according to godliness; He is proud, knowing nothing, but doting about questions and strifes of words, whereof cometh envy, strife, railings, evil surmisings, Perverse disputings of men of corrupt minds, and destitute of the truth, supposing that gain is godliness: from such withdraw thyself.
II John 10-11: If there come any unto you, and bring not this doctrine, receive him not into your house, neither bid him God speed: For he that biddeth him God speed is partaker of his evil deeds.
Matthew 7:6: Give not that which is holy unto the dogs, neither cast ye your pearls before swine, lest they trample them under their feet, and turn again and rend you.
While the people of Israel were commanded not to listen to false prophets, we are to listen in order to find out if they are open to the gospel. We are to pray for their salvation and try to lead them to faith in Jesus Christ. It is hard to know at times whether or not to proceed with sharing the gospel with them. One good method to find out is to simply ask them the question, If what you believe were not true, would you want to know it? If they say yes, they may be open enough. If they say no or beat around the bush, forget about leading them to the Lord for the time being.