In Daniel 10:12-14, there is a spiritual warfare due to Daniel's "words," but not his thoughts. Then he said to me, "Do not be afraid, Daniel, for from the first day that you set your heart on understanding this and on humbling yourself before your God, your words were heard, and I have come in response to your words. But the prince of the kingdom of Persia was withstanding me for twenty-one days; then behold, Michael, one of the chief princes, came to help me, for I had been left there with the kings of Persia. Now I have come to give you an understanding of what will happen to your people in the latter days, for the vision pertains to the days yet future.”
This passage tells us that from the first day Daniel prayed and his words were heard by God. We read that God commissioned Gabriel to come to him. Now…who is this prince of the kingdom of Persia? This refers to an evil angel appointed by satan to control the Persian Government, which was done of course, without the knowledge of its earthly king. So, this passage appears to reveal that satan places an agent in charge of every nation - that is a fallen, evil angel. This explains why we see and read so much of national hatreds and national movements.
Scripture is clear that God has His Angelic Angels operating in opposition to satan’s evil angels. Also, the Bible is clear that only God is ever mentioned as knowing the thoughts of man. Consider what the Bible says in 1 Chronicles 28:9 "As for you, my son Solomon, know the God of your father, and serve Him with a whole heart and a willing mind; for the LORD searches all hearts, and understands every intent of the thoughts. If you seek Him, He will let you find Him; but if you forsake Him, He will reject you forever." See also Psalm 139:23, Isaiah 66:18, Ezekiel 11:5.
Satan certainly can hear you, and he knows your sin nature as he is your natural "father" (before conversion). Remember John 8:44 tells us "You are of your father the devil, and you want to do the desires of your father. He was a murderer from the beginning, and does not stand in the truth because there is no truth in him. Whenever he speaks a lie, he speaks from his own nature, for he is a liar and the father of lies." In this capacity, satan knows where to tempt you and he knows where you are weak.
The main thing to understand and remember to encourage us today is that God, living powerfully in you, is much greater than Satan and your sin nature. Therefore, there is absolutely nothing to fear, even if you are praying out loud.. . . greater is He (Jesus) who is in you than he (Satan) who is in the world. —1 John 4:4