This question was submitted by someone whose concern primarily had to do with prayer.  That is, if demons can read our minds, should we worry about conscious thoughts which identify our areas of weakness (allowing the evil one to use that information against us)?  On the other hand, if demons cannot read our minds, ought we weigh careful what weaknesses we admit to aloud in our prayers (also for fear that we would be arming the enemy with critical information of our struggles only to be used against us in temptation)?   

Who knows the heart?
I’m assuming here that what we speak of as the ability to “read another’s mind” would be equivalent to the biblical concept of “knowing the heart” of a person.  That is, having access to even the secret thoughts and intentions of the deepest recesses of the soul.  When we consider the array of Scripture, the knowledge of such “secret” heart-information seems to be limited to God alone (Ps 94:11; Ps 139:2-4, 23; Pr 16:2; Jer 12:3).  The prophet Jeremiah, recognizing the hiddennes of some heart-information (apparently even hidden from the person who possesses it) writes, “The heart is more deceitful than all else and is desperately sick; who can understand it? (Jer 17:9).  Yet, we also read that it is God alone who “searches every heart and understands every motive behind the thoughts” (1 Ch 28:9).  But what about demons—which are merely fallen angels—what do they know of such secret heart-information?

The knowledge of angels—fallen and otherwise
The Apostle Peter, speaking of God’s secret plan to repair His sin-broken world through the person of Jesus, writes,

“10 Concerning this salvation, the prophets, who spoke of the grace that was to come to you, searched intently and with the greatest care, 11 trying to find out the time and circumstances to which the Spirit of Christ in them was pointing when he predicted the sufferings of the Messiah and the glories that would follow. 12 It was revealed to them that they were not serving themselves but you, when they spoke of the things that have now been told you by those who have preached the gospel to you by the Holy Spirit sent from heaven. Even angels long to look into these things” (1 Pet 1:10-12). 

Note the last sentence in these verses.  This seems to place angelic beings in the category of humans, with respect to their inability to access information in another’s mind (in this case, God’s mind).  They (both angles and demons), like humans, did not have access to the secret thoughts of God, and therefore, didn’t know the details of how God would carry out the restoration of a wrecked creation.

So, if it is true that angels don’t have access to thoughts, how would they know so much about us?  I’d suggest it is because Satan and his minions are careful students of human nature.  They observe and remember our responses in various scenarios.  Imagine if you could watch and listen to someone all throughout his or her day—even the unguarded moments in the car or behind closed doors.  It wouldn’t be long before you’d know appetites, dislikes, tendencies, what things get a second or even third look, and much more.  The Bible sometimes refers to demons as “familiar spirits,” in that they are familiar with us, having studied us.

Then ought we guard our prayers spoken out loud?
In light of all this, the question might then be, “So, should we now worry about demons hearing our spoken prayers?”  With all that’s written about prayer in the Bible, never is there even an ounce of suggestion that we out to be concerned about what we say aloud to God (in confession of sin or otherwise).  Jesus’ only discussion of ‘secret’ prayer was directed to those “hypocrites” who would pray “standing in the synagogues and on the street corners to be seen by men” as super-spiritual (Mt 5:5-6).  Jesus not only instructed his apprentices to be prayerful, but he also taught them how to go about it, and corporate, out loud prayers were a part of that training (Mt 5:44; 6:5-15; Lk 11:1-13; 18:1-8). 

Two opposite extremes
I can only imagine that paranoia about either (1) our prayers being said aloud, or (2) being known by demons via “reading one’s mind”, would be precisely what the enemy would desire.  If Satan can’t get you to believe the lie that your prayers are simply not being heard by God at all, deceit in the opposite direction will suffice just as well; “You mustn’t pray for fear that the wrong spirit will hear!”  Prayer which creates fear or concern is the opposite of biblical prayer.  Rather, biblical prayer is communication with God, the effect of which is pictured in the Bible as our worries and anxieties being literally thrown off our shoulders and onto God’s (1 Pet 5:7).  Biblical prayer functions to re-center the self in the greatness of God—the only place where final peace can be found.  The follower of Jesus who comes to God in honest prayer, whatever the nature of it, has absolutely nothing to worry about from the enemy.