I’ve seen various memes about us and the devil on social media recently, along the lines of the one displayed above. The purpose of them is obviously to give us a certain confidence in God and his power over evil; some of them cross the line, however, making it sound like we can somehow contend with the devil. That’s just dumb — and dangerous.
In any case, there is a certain truth conveyed by the meme in this post, and that is: the devil cannot read our thoughts.
He is far more intelligent than we — or than we can really fathom.
He observes us intently and can deduce many things from what he observes.
But he cannot see with certainty what we are thinking, if it remains only in the realm of thought and we do not visibly express it.
St. Thomas Aquinas teaches this in his Disputed Questions on Evil (De Malo), question 16, article 8. And the Second Vatican Council teaches us (Gaudium et spes 16) that our conscience is our most secret core, where we are alone with God.
So the above meme is correct: when we pray (when we are not moving our lips or otherwise manifesting the content of our prayer!), the devil does not know what we are saying to God. The evil one cannot read our thoughts. We are safe from him there. When we pray, therefore, we must speak to God openly, honestly, and with confidence — without fear that the devil might be listening in!