I was always taught that satan could not read our thoughts but could hear what we say. I'm careful about what I say in general because I know that the power of life and death are in our words. But I'm wondering; if we pray out loud, does this give satan more information about how he can target us? I'm certainly verbal when I rebuke him and that is effective.
- LJ, City unknown
Fewer is he (Part 1)
I have had similar thoughts at different times over the years so I understand your question. Since we do not live in the same spiritual realm Satan does and since the Bible does not really speak to this issue (at least not that I can find so far) we have to make some reasonable assumptions.
Let's start with what we know for sure. Satan is a created being so he is not all powerful, all knowing or all present. He is not just a little less than God; he is light years away from the One who brought him into existence. Since he can't be everywhere at once he can't possibly be listening in on everyone all over the world.
Well, what about the rest of the fallen angels? There probably aren't enough of them to go around either. Our indication from Revelation 12:4 is that when Satan rebelled and was cast out of heaven about one third of the angelic force went with him. "And his tail swept away a third of the stars of heaven, and threw them to the earth." Some of them have been chained up in a pit and will be released during the Tribulation so not all are available as listening posts anyhow.
Of course we do not know how many angelic beings were originally created by God. However, if one third fell, there would still be two thirds left who serve God. And what do they do? "Are they not all ministering spirits, sent forth to minister for them who shall be heirs of salvation?" (Hebrews 1:14)
I know of a person who has the gift of discerning of spirits. She has often seen or sensed angels surround people as they pray. Perhaps part of their job as Guardian angels is to protect those in their care as they pour out their heart to the Lord. I have no biblical back up for that but it is one of those assumptions that makes sense to me.
Does he use our prayers against us? (Part 2)
We are reminded in scripture to "be of sober spirit, be on the alert. Your adversary, the devil, prowls about like a roaring lion, seeking someone to devour." (1 Peter 5:8) Even if he or his minions don't observe your every move they certainly observe you sometimes. So, if they hear, will they use your prayers against you?
You noted in your question that you are aware that the power of life and death is in the tongue. You generally watch what you say. I think that is a good position to take as you pray, whether or not Satan is listening in.
Even when we are being honest before the Lord and saying things we would not want him to hear, we can pray in faith and trust in God. We can start our prayer time with praise and worship (which Satan would find abhorrent - maybe he would cover his ears). We can affirm that we are bondservants of the Most High God and we can plead the blood of Jesus over every part of our lives. We can ask God to put a hedge of protection around us as we pray. We can ask him to assign angels to protect us. We can even start by binding the devil from listening and demanding that he leave if he is anywhere near.
Finally, we shouldn't worry too much about who is listening while we pray. Our actions probably speak louder than our words and those are out there for angels and demons to observe. In the end, the fact that we ARE praying is stronger and more effective than any possible plot to trip us up. The most important thing to remember is not whether the devil hears us; the most important thing is that God does.