In addressing Satan’s abilities, let’s start with his inabilities. As an angel, the devil is not omnipresent. Only God the Creator is (Psalm 139:7-10). Much as the human soul, housed in a body, can occupy only one space at one time, so the devil, a spirit being, can occupy only one space at one time.
Yet, as a spirit being without a body, the devil is able to move from one space to another faster than human beings can. And move he does. Job 1:7 and Job 2:2 describe the devil “roaming throughout the earth.” 1 Peter 5:8 states that the devil “prowls around like a roaring lion looking for someone to devour.” Roaming and prowling describe the devil’s non-stop activity as he moves from person to person, seeking to destroy them.
With this kind of activity, Satan and his helpers could hear our conversations. Then, again, keep in mind that Satan is not all-knowing as God is. Satan cannot read hearts and minds. Only God can do that (1 Kings 8:39). Satan operates by what he observes and hears.
It is good to be aware of Satan’s power and limitations. Here is something else to keep in mind: we want to recognize that the devil has an accomplice inside each person. The sinful nature, passed on from generation to generation, is hostile toward God (Romans 8:7). The sinful nature hates anything good and godly and desires to do the devil’s bidding (Galatians 5:19-21). So while the devil is not personally present in the life of a Christian 24/7, his ally, the sinful nature, is.
With spiritual enemies like these, what is a Christian to do? “Put on the full armor of God” (Ephesians 6:13) and launch a counter-offensive. That is not a foolhardy course of action. “Resist the devil, and he will flee from you” (James 4:7) is God’s instruction and promise. More than that, we combat our sinful nature through daily contrition and repentance, and we build up the new self through God’s gospel in Word and sacrament (Ephesians 4:22-24).