|Jeremiah lamenting the fall of his people|
Jeremiah was the most famous prophet at the time but there were others, some real some false. Judah was filled with so called prophets who preached that all was well in Judah. They were nationalistic and led people astray. They tolerated idolatry and "secret places" in Judah. They preached about Baal and told the people they had dreams about Baal. Sure they were priests of God, but Baal is cool too. If you read Jeremiah 23, the Lord lists the lies these so called prophets were sharing. In Jeremiah 28, a false prophet is mentioned by the name of Hananiah. Hananiah began to preach a little after Lehi and his family left Jerusalem. He taught that within two years Nebuchadnezzar, King of Babylon would break the yoke of wood around Judah's neck. Jeremiah was told to tell Hananiah that "I have put an yoke of iron upon the neck of all of these nations, that they may serve Nebuchadnezzar, king of Babylon; and they shall serve him: and I have given him the beasts of the field also." (Jeremiah 28:14)
That was the atmosphere that Lehi lived in. One where false prophets preached nationalism and secularism against the Lord's warnings. Lehi's elder sons, Laman and Lemuel were certainly influenced for in 1 Nephi 2:13 it says, "Neither did they believe that Jerusalem, that great city, could be destroyed according to the words of the prophets." Lehi and Jeremiah preached what was unpopular even if it was the truth. Jerusalem fell not long after they prophesied.
The Bible warns of false prophets. False prophets aren't going to preach something strange at first. There are weird cult leaders who attract the weak and naive but the best false prophets are going to make you feel good and proud, even while you sin. They preach that because you are so special, you can't possibly fail.
Ways to know if a prophet is false- Deuteronomy 18:22 If he prophesies something and it doesn't come to pass. One such false prophet that comes to mind is Karl Marx and Vladimir Lenin who prophesied a great class war. Even though groups are trying hard to make the prediction true, it never does. Global warming is another such prophesy. Florida was supposed to be underwater and the ice caps were supposed to melt. Well, it has become colder rather than hotter. False prophets don't have to represent a church!
(D&C 42:11-12) If someone was not given the authority they cannot be a prophet. They might receive revelation for themselves or their family, but they do not prophesy for a people unless they are given that authority from God. When you hear of Mormons getting excommunicated for leading movements in the LDS church, it was for this reason. They made themselves a prophet, or leader without authority from God. Also, a true prophet will preach the Gospel and witness for Jesus Christ and not preach anything contrary to the scriptures. Nationalism doesn't fall in that category. (what the priests of Zedekiah were guilty of)
We are warned that many false prophets will come in the Last Days. (Matthew 24: 3-8) What will they preach? 2 Timothy 4: 3-4 explains- "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 their ears away from the truth." We live in a similar time to Lehi and his family.
M. Russell Ballard says it better "Beware of False Prophets" 1999