First, neither the Devil nor his demons know your thoughts whatsoever. If He did, then He would have known all Jesus’ thought, the plan of crucifixion and the ultimate demise of himself to be utterly defeated. What does he know? He knows a lot, to be perfectly straightforward. He has been in the human personality business for thousands of years and knows the action and reactions of man's personality. He knows what has been written in the bible, he knows his end, he has access to heaven (not in God's government ) for he still is a spirit and can move around without you or anyone seeing him, except The Trinity and all of God's angels.

Most importantly he knows the authority we have when we believe and have faith in the word of God. He knows we can cast out demons and sickness he puts on others and us. He can oppress you by sending thoughts, fear and planning death traps, accidents and such, but doesn't know you receive the oppression until you give evidence that you did. For instance, he puts fear in your mind and you start acting fearful and speaking it, he then knows and brings more, as oppression, you speak out against fear in Jesus name and bind it, it goes away, he is defeated, only to try again sometime later on.

This is the warfare, he accuses, brings death and destruction and sickness, God brings life, victory and everlasting eternal life and has total power over The devil not only in Heaven but through you as a believer. God of course knows your thoughts and gives you the desires of your heart, instruction both when you are awake and in your sleep, He establishes your footsteps by guiding your thoughts that control your actions. Your Christian walk is fellowship constantly with the Holy Spirit by conversation minute by minute for your whole life. There are many scriptures; I will list but a few below.

See: John 10:10, Matthew 28:19, 1 Corinthians 8:6, 2 Corinthians 13:14, Matthew 3:16-17, John 14:26, John 1:14, 1 John 5:7-8, John 14:14-17, 1 Peter 1:2.