Jesus warned about a massive deception to take place in the last days more than any other sign.....
Take heed that no one deceives you For many will come in My name saying "I am the Christ" and will deceive many......Then if anyone says to you, "Look, here is the Christ" or "There!" do not believe it. For false christs and false prophets will rise and show great signs and wonders to deceive, if possible, even the elect. See I have told you beforehand. Therefore, if they say to you "Look He is in the desert!" do not go out, or "Look, He is in the inner rooms!" do not beleive it! For as lightening comes from the east and flashes to the west, so also will the coming of the Son of Man be. Matthew 24: 4, 23-27
Beloved, 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 1 John 4:1
Preach the word; be instant in season, out of season; reprove, rebuke, exhort with all long-suffering and doctrine. 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 to fables. Be sober in all things 2 Timothy 4:2-4
I marvel that you are turning away so soon from Him who called you in the grace of Christ, to a different gospel, which is not another, but there are some that trouble you and want to pervert the gospel of Christ. But even if we, or and angel from heaven, preach any other gospel to you than what you have reveived, let him be accursed. As I have said before, so now I say again, if anyone preaches any other gospel to you than what you have received, let him be accursed. Galatians 1:6-9
For such are false apostles, deceitful workers, transforming themselves into apostles of Christ. And no wonder! For Satan himself transforms himself into an angel of light. Therefore it is no great thing if his ministers also transform themselves into ministers of righteousness, whose end will be according to their works 2 Corinthians 11:13-15
But I fear, lest somehow, as the serpent deceived Eve by his craftiness, so your minds may be corrupted from the simplicity that is in Christ, For if he who comes preaches another Jesus whom we have not preached, or if you received, or a different spirit which you have not received, or a different gospel which you have not accepted -- you may well put up with it. 2 Corinthians 11: 3-4