I am reading William Bouwsma's excellent portrait of John Calvin. Bouwsma emphasizes that Calvin was a rhetorician, not a cool scholar. He was a pastor, trying to persuade his listeners to change their lives. Calvin also read the Bible as rhetoric of the same kind.
One of the main tools of persuasive rhetoric is to exaggerate the dangers that the audience faces if they continue their present lives.
In my work on happiness, I have concluded that the main solvent of the happy society is fear. Fear mongers are a great danger to a happy society, because they undermine trust, and obscure how much the good actions of most people make the world better.
Which leads me to look for a religious limit to fear-mongering. There is, of course, the general commandment against lying. But I can't think of a specific religious doctrine or practice the guards against overstating the dangers of this world.
Overstating dangers is bad for the credibility and legitimacy of religious organizations, as we can see from the short life-span of doomsday cults. As a practical matter, most religious institutions that last more than a couple of generations do learn to tone down the end-times rhetoric, and start to build for the indefinite future.
Still, I can't think of a religious justification that I have run across for telling the strict truth about dangers and fears.