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False prophets are those who falsely claim to have a message or teaching from God. There has never been a shortage of people prepared to deceive others in this way.
Matthew 24:24 For there shall arise false Christs, and false prophets, and shall shew great signs and wonders; insomuch that, if it were possible, they shall deceive the very elect.
2 Peter 2:1 But there were false prophets also among the people, even as there shall be false teachers among you, who privily shall bring in damnable heresies, even denying the Lord that bought them, and bring upon themselves swift destruction.
By the end of the 4th century AD the Christian Church had sunk into apostasy due to false prophets and teachers.
Some false prophets have established their own none-Christian religions.
Others have introduced modified versions of Christianity.
A relatively recent example of the latter would be Joseph Smith and the book of Mormon.
Those who today claim to have Holy Spirit gifts are all false prophets.
With the completion of the New Testament the gospel and doctrine was fixed and no further revelation is necessary. All that we need to know for salvation is written down in the Bible. It is written down to prevent us being deceived.
2 Timothy 4:3 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;
The doctrine is fixed.
1 Timothy 4:16 Take heed unto thyself, and unto the doctrine; continue in them: for in doing this thou shalt both save thyself, and them that hear thee.
2 John 1:10 If there come any unto you, and bring not this doctrine, receive him not into your house, neither bid him God speed:
The gospel is fixed.
Galatians 1:8 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.
We can and must compare whatever is preached with that which is written.
2 Timothy 3:16 All scripture is given by inspiration of God, and is profitable for doctrine, for reproof, for correction, for instruction in righteousness: 17 That the man of God may be perfect, throughly furnished unto all good works.
I hope you have found this helpful.
May God bless you,