Francis gives only one example of fake news in his treatise. He is the Pope, so no surprise, it's from the Bible.
"This was the strategy employed by the 'crafty serpent' in the Book of Genesis, who, at the dawn of humanity, created the first fake news," Francis wrote. He means the serpent in the Garden of Eden, who tricked Eve and Adam into eating forbidden fruit by making up a story about how great it would turn out.
"The tempter approaches the woman by pretending to be her friend, concerned only for her welfare, and begins by saying something only partly true," Francis wrote. " 'Did God really say you were not to eat from any of the trees in the garden?' "
False premise. "In fact," Francis wrote, "God never told Adam not to eat from any tree, but only from the one tree."
Eve tries to correct the serpent, and in doing so, falls for his trap. It's a bit like when you argue with a Facebook troll and get sucked into a long comment thread, eventually saying things you never meant to.
"Of the fruit of the tree in the middle of the garden, God said, 'You must not eat it nor touch it, under pain of death,' " Eve tells the serpent, very specifically.
"Her answer is couched in legalistic and negative terms," Francis wrote, "After listening to the deceiver and letting herself be taken in by his version of the facts, the woman is misled. So she heeds his words of reassurance: 'You will not die!' "
And then, like with a chain email, Eve shares the serpent's news with Adam, who turns out to be just as gullible. And while they don't die when they eat the fruit, they do get the human race kicked out of paradise forever.
That's how fake news worked back in Genesis, Francis wrote, and it's not much different and no less dangerous in the internet age.
So, he asked, "How can we recognise fake news?"
He listed a few characteristics of the genre: Fake news is malicious. It plays off rash emotions like anger and anxiety. "It grasps people's attention by appealing to stereotypes and common social prejudices," Francis wrote.
But in most respects, fake mimics truth. On the surface, they can be hard to tell apart. For example Trump once retweeted a video titled "Muslim migrant beats up Dutch boy on crutches!" The video was real, but police said the attacker wasn't even a migrant.
So finally, here is the Pope's solution. "We can recognise the truth of statements from their fruits," he wrote, "whether they provoke quarrels, foment division, encourage resignation; or, on the other hand, they promote informed and mature reflection leading to constructive dialogue and fruitful results."
Fake news is as fake news does, in other words. It "leads only to the spread of arrogance and hatred," Francis wrote.
So if you're feeling those things while browsing Facebook, or find yourself in a flame war, be especially wary of what you just read. Ask yourself if there might be another side. Listen to those who disagree with you, instead of yelling at them.
"The best antidotes to falsehoods are not strategies, but people," the Pope wrote. "People who are not greedy but ready to listen, people who make the effort to engage in sincere dialogue so that the truth can emerge; people who are attracted by goodness and take responsibility for how they use language."
If you're wondering, no, the Pope does not mention Trump in this message. Not that Francis mentioned him by name either during the 2016 campaign, when he told reporters, "A person who thinks only about building walls, wherever they may be, and not building bridges, is not Christian."
But the contrast between these two men's notions of fake news is glaring. If Trump's appeals, you can find it on his Twitter account. If what Francis wrote makes sense to you, you might try it out the next time your scroll through Twitter.
Ask yourself if what you read makes you feel hateful, or like quarrelling. Ask if the Pope might find it fake.
And you could ask the same of everything you read, including this article, which brought Trump into the Pope's message, even though the Pope did not.
Indeed, Francis wrote toward the end of his essay, "If responsibility is the answer to the spread of fake news, then a weighty responsibility rests on the shoulders of those whose job is to provide information, namely, journalists, the protectors of news."
Just as everyone should check their emotions against the news, he wrote, the news should avoid inciting them.