Your prophets have seen for you
false and deceptive visions;
they have not exposed your iniquity
to restore your fortunes,
but have seen for you oracles
that are false and misleading.
Not only does Jeremiah describe the physical devastation of Jerusalem along with the personal tragedy of death, starvation and homelessness in the streets, but he also intersperses his mourning with insights into the spiritual deterioration of the nation. For generations the Israelites believed what they wanted to be true. They trusted in themselves. They listened to the "positive and encouraging" messages of false prophets who made them feel good in their continued sin.
There was a lot of spiritual activity in Jerusalem. It was just the wrong kind. Idolatry was rampant for generations. The worship of The Lord, when maintained, was more of a cultural remnant than a living commitment. Yahweh had become another god among many. And for the most part the majority of the prophets and spiritual leaders did not expose this for the evil that it was.
The false prophets deceived the people. And deception's worst pain comes when the big lie is uncovered. But by then it was too late for the exiled generation. Families were destroyed. Children starved in the rubble of Jerusalem's broken and burned gates. Spiritual lies will take a massive toll on humanity... even ones that seem so enlightening. Of all the tears shed in the book of Lamentations, I think the most bitter ones fell at the realization of the layers of lies woven by the false prophets. It could have all been avoided by real trust in the covenant and in The Lord of Israel. But leaders lied, twisted the truth, and many people died in the unraveling of it.