1 John 4:1-4 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. 2 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, 3 but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the antichrist, which you have heard is coming and even now is already in the world.
John 3:16-18 For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life. 17For God did not send his Son into the world to condemn the world, but to save the world through him. 18Whoever believes in him is not condemned, but whoever does not believe stands condemned already because he has not believed in the name of God's one and only Son.
That is the issue. What do they think, teach and believe about Jesus. No matter how nice they look, preach or teach, if they deny this fundamental doctrine, turn away and never look back.
2 Timothy 3:1-5 This know also, that in the last days perilous times shall come. 2 For men shall be lovers of their own selves, covetous, boasters, proud, blasphemers, disobedient to parents, unthankful, unholy, 3 Without natural affection, truce breakers, false accusers, incontinent, fierce, despisers of those that are good, 4 Traitors, heady, high minded, lovers of pleasures more than lovers of God; 5 Having a form of godliness, but denying the power thereof: from such turn away.
Isaiah 14:12-14 "How you are fallen from heaven, O Lucifer, son of the morning! How you are cut down to the ground, You who weakened the nations! 13 For you have said in your heart: "I will ascend into heaven, I will exalt my throne above the stars of God; I will also sit on the mount of the congregation On the farthest sides of the north; 14 I will ascend above the heights of the clouds, I will be like the Most High.'
As a result of this act of rebellion Satan was thrown out of heaven, along with thousands of other angels who rebelled with him. This truly began the "new age" or the replacement of worshipping the one true God by worshipping something or someone else.
So, since this great rebellion, Satan and his fallen angels are using false teachers to promote another gospel that denies God and His Son, Jesus Christ.
Galatians 1:8-9 But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed. 9 As we said before, so say I now again, if any man preach any other gospel unto you than that ye have received, let him be accursed Paul again warns us of the sinister intentions of false teachers and their motives to deceive us.
2 Corinthians 4:3-5 But if our gospel be hid, it is hid to them that are lost: 4 In whom the god of this world hath blinded the minds of them which believe not, lest the light of the glorious gospel of Christ, who is the image of God, should shine unto them. 5 For we preach not ourselves, but Christ Jesus the Lord; and ourselves your servants for Jesus' sake.
2 Peter 2:1-3 But there were also false prophets among the people, just as there will be false teachers among you. They will secretly introduce destructive heresies, even denying the sovereign Lord who bought them--bringing swift destruction on themselves. 2 Many will follow their shameful ways and will bring the way of truth into disrepute. 3In their greed these teachers will exploit you with stories they have made up. Their condemnation has long been hanging over them, and their destruction has not been sleeping.
Lastly, the Apostle Paul again warns the church at Corinth that we shouldn't be surprised when this happens. When Satan fell from heaven his one and only goal is to deceive mankind into worshipping anything except Jesus Christ. That is the root of the problem of false teaching as it denies the divinity of God's plan to redeem the world through His Son, Jesus.
2 Corinthians 11:3-4 But I am afraid that just as Eve was deceived by the serpent's cunning, your minds may somehow be led astray from your sincere and pure devotion to Christ. 4 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 one you received, or a different gospel from the one you accepted, you put up with it easily enough.
Matthew 24:23-24 At that time if anyone says to you, 'Look, here is the Christ!' or, 'There he is!' do not believe it. 24 For false Christs and false prophets will appear and perform great signs and miracles to deceive even the elect--if that were possible.
If the early church was warned against false teaching 1900 years ago then the warning still holds for today. As we come closer to the return of Jesus Christ it will get worse, so be ready and don't be fooled by those. They will receive their just reward as Paul wrote to the church at Corinth.
2 Corinthians 11:13-15 For such men are false apostles,
deceitful workmen, masquerading as apostles of Christ. 14And no wonder, for
Satan himself masquerades as an angel of light. 15It is not surprising, then, if
his servants masquerade as servants of righteousness. Their end will be what
their actions deserve.
As the Apostle Paul wrote again to the church at Corinth.
1 Corinthians 16:21-22 The salutation of me Paul with mine own hand. 22 If any man love not the Lord Jesus Christ, let him be Anathema Maranatha.
So be smart and don't be fooled by anyone who is teaching another gospel other then the one that you have been taught. Test the spirits, listen carefully and if deny Jesus, turn away, run and never look back.