I'm sure I understand your comment or what is behind it. Can you explain, please?

Since 300ad

The harm is that people are believing a lie from the mouth of someone who is pretending to be authorized by God to be telling God's truth. So this person takes the liar as a man of God, sworn to tell the truth, and to be telling us what God Himself says, when it's all a lie. It's a most insidious perversion.

What harm comes being deceived by a false prophet?

Matthew 24:24 says: "For false messiahs and false prophets will appear and perform great signs and wonders to deceive, if possible, even the elect."

Ignoring for a moment the issue of false Christs, I'd like to focus on false prophets.

What happens when a people follow a false prophet? Many Christians view Mormons and Jehovah's Witnesses as having been led away by a false prophet. Where is the harm in this?