They speak visions of their own minds.

Thus says the Lord of hosts: “Do not listen to the words of the prophets who prophesy to you, filling you with vain hopes. They speak visions of their own minds, not from the mouth of the Lord. They say continually to those who despise the word of the Lord, 'It shall be well with you’; and to everyone who stubbornly follows his own heart, they say, 'No disaster shall come upon you.’”
Jeremiah 23:16-17

An overwhelmingly enthusiastic optimism that ignores the Word of the Lord is the mark of a false prophet. They lying false prophet, smiling and telling you that your life will be well and blessings are yours for the taking right now is denounced by God. What's worse, those who are attracted to this false optimism have abandoned the entire counsel of God to follow a glittering lie.

The problem is that the false optimism does not speak the truth completely. It might actually have little snippets of the truth mixed with the error, but it draws the focus away from God, and onto human beings. Yes, it is true that God loves us and wants to work in our lives. But we are born sinners. Grace means that we don't deserve the good news of the gospel. We have to accept the bad news along with the good news. Awareness of sin must leave us desperately wanting to repent as the Word of God confronts all our fallen faults. Only then can real hope, change, and joy be found in repenting and believing the good news.

I wish that God only had to take Judah to task for this in the Old Testament and that the church was never guilty of embracing false teaching. Just before the fall of Jerusalem, false prophets undermined Jeremiah's message with their overwhelming and wildly acclaimed prosperity theology optimism. And Christians in my day, in my culture, seeking distraction from the hard work of incarnating the gospel in a pagan society, soak up the same lame saccharine drivel. It comes from the pastor of the largest congregation in America located in Houston, Texas. From a former NBA stadium, a smiling, smarmy, well-coiffed and immaculately tailored salesman, that frankly gives me a case of the willies, a man who never went to seminary and proudly admits it, who owns up to not be well versed in any Bible doctrine in national television interviews, feeds self-help sugary sermons of health and wealth nonsense to millions of adoring fans who buy his books that always have his smiling face on the cover. What tastes so good to them is soul poison. Let the Osteens and the Oprahs of this world have their adoring soul zombie army. Their message is doomed when eternity looms. Those who stubbornly follow their own hearts will find out the deception much too late. I'm afraid though that in the last day some will hear Jesus say, "I never knew you". That is the ultimate tragedy of false teaching.