This blog was taken from the Gospel Coalition website. This blog was written by Joe Carter is an editor for The Gospel Coalition and the co-author of How to Argue Like Jesus: Learning Persuasion from History's Greatest Communicator. You can follow him on Twitter.
The Bible doesn't directly address the ways that demons know about humanity, but I think we can find a clue in Job 1:6-7: "The Lord said to Satan, 'From where have you come? Satan answered the LORD and said, 'From going to and fro on the earth, and from walking up and down on it.'" As fallen angels, demons possess intelligence, express emotion, and exercise will. They also have a broad range of experience from directly observing and dealing with humanity for eons. As S. Michael Houdmann says,
[A]ngels gain knowledge through long observation of human activities. Unlike humans, angels do not have to study the past; they have experienced it. Therefore, they know how others have acted and reacted in situations and can predict with a greater degree of accuracy how we may act in similar circumstances.
So the short answer is "watching what humans do," but I think by considering the question as a "black box problem" we can gain a better understanding of how demons know so much about us.
Demons as a Black Box Problem
A black box problem is a closed system that allows us no knowledge about its functionality (e.g., some scientists consider the functioning of the brain/mind to be a black box problem). We can't open up the box and look inside to see how it works, though we can see what happened when something was done to it (input), and what occurred as a result of that (output).
For example, let's imagine we put an apple into a hole on one side of a black box and on the opposite side, out comes applesauce. If we have no experience with applesauce (and limited knowledge about apples), the workings of the box may appear to be a complete mystery. But let's assume we once saw our grandmother make applesauce by mashing up apples in her kitchen. We can assume that the process inside the black box is likely to be similar to grandma's method.
Knowing how it could be done increases our understanding of the process even if the true method is much more mysterious, sophisticated, or complex. For instance, imagine the real process entails a wormhole inside the box that sends the apple to a parallel universe where it is mashed by the hoof of a unicorn and teleported back as applesauce. Even in such a strange case, by knowing granny's method and applying analogical reasoning, we can better comprehend the process in the box.
How Demons are Like the NSA—and Your Grocery Store
The question of how demons know about us can illuminated, I believe, by comparing it to how your grocery store, Amazon, Google, or intelligence agencies like the NSA all use metadata to know our habits.
Take, for instance, the grocery store "loyalty card" that most of us carry. By using the cards, we get discounts on groceries, and the store gets detailed information about our shopping habits. Every transaction you make at the store can be tracked and stored in a computerized database. The metadata is the "data about data content" the store knows about you. When you swipe your loyalty card, the store's computer can know what you purchased (based on the price barcode), the time you bought an item (based on the computer's internal clock), what other items you bought at the time, and various other information about you. The store not only knows where you live and what you buy, but could, based on the metadata collected in the past, even predict that on Friday evening you will buy Pop Tarts and Diet Coke.
The information collected by a grocery store is rather crude. But by adding a few more pieces of metadata (e.g., your Social Security number, zip code), the store could estimate fairly accurately when you were born, how much money you make—and even how much you weigh.
What Demons Could Learn By Watching You
Now let's apply this concept analogically to the black box problem of how demons know information about us. The NSA and your local supermarket use powerful computer systems to collect and track metadata and develop crude profiles about you and me. Presumably, demons don't have computerized databases, though let's assume they have some method of collecting, tracking, and sharing information throughout their vast demon network (in other words, they share notes with other demons). They also have an advantage over intelligence agencies in that they can observe our behavior directly.
Consider what an intelligent being with an expansive ability at data collection could learn by simply observing you over a 24-hour period. Imagine that they can see not only the content of the e-mails you type, but also what you wrote and erased before hitting the send button. Imagine they took notes on what websites you visited, how long you stayed on a page, and the facial expressions you made while reading the content. Imagine also they listened to your cell phone conversations and overheard you gossiping to your spouse about a coworker.
What sort of profile could the demon-agent assigned to you build by just watching you for a day? How much could they know if they had been observing you since birth, tirelessly taking meticulous notes? Imagine if the demon network had collected more information on you than could be contained in Google's servers.
Satan's Useless Knowledge
Knowing that demons can know so much about us should make us grateful that we are in Christ and that "he who is in you is greater than he who is in the world" (1 John 4:4). The demon network may harass us (2 Corinthians 12:7), but the mountain of metadata they may have on us is useless if we use the power given us by the Holy Spirit to resist their temptations (James 4:7).
Though we should not personally fear Satan's minions, we should still be concerned about the influence they have in the earthly realm. Those of us who—like Jesus—take the existence and activities of demons seriously will want to make their job much more difficult by leading others to Christ, the only one who can lead us away from temptation and deliver us from the Evil One.