Question:
Does the devil hear your silent prayers, and know what you are praying for? Can he read your mind?

Answer:
I know of no Scripture which would lead us to believe that the devil can read our minds (which would include hearing our silent prayers.) Only God has such knowledge, as is evident from the following Scriptures: “O LORD, thou hast searched me, and known me. Thou knowest my downsitting and mine uprising, THOU UNDERSTANDEST MY THOUGHT AFAR OFF . . .  Search me, O God, and know my heart: try me, AND KNOW MY THOUGHTS” (Psalm 139:1, 2, 23).

The devil does have the power to put thoughts into one’s mind, for we read in John 13:2, “And supper being ended, THE DEVIL HAVING NOW PUT INTO THE HEART OF JUDAS ISCARIOT, SIMON’S SON, TO BETRAY HIM” (see also Acts 5:3), but thank God he is, as far as I know, unable to discern the thoughts that we have when not under his influence.