Can Satan and his demonic followers directly plant thoughts into the minds of believers in the Lord Jesus Christ?
I have been asked this question many times over the years, and my honest response is that I have found no definitive Biblical answer to the question. It certain feels and seems that our spiritual enemies are allowed such inroad into our thinking. However, I lean toward the perspective that the devil must have fleshly and worldly cohorts in order to present us with temptation, namely the world and the flesh.
Recall that in the original temptation of man, Satan took the form of a serpent in order to tempt Eve. Subsequently, Adam was tempted through Eve (Genesis 3:1-6). Environment and people thus served as the devil's means of presenting his deception to the woman, and fleshly enticement to the man. According to the Apostle Paul, Eve was deceived, and Adam's sin was willful because he "was not deceived" (I Timothy 2:14). In both cases, however, temptation began from without as the devil used outward influences to lure humanity into his fallenness.
Presently, a "law of sin" exists within the flesh of believers (Romans 7:22-25). This was not the case with Adam and Eve before they distrusted and disobeyed God. There is a difference, therefore, in our experience and that of our original forefathers. Perhaps Satan has inroads into our consciousness that he did not have with Adam and Eve. Again, it seems and feels this way, and if I one day discover this to be the case Scripturally (or when we get to Heaven and are given greater insight), I won't be surprised. I suspect, however, that outward influences such as the world and the flesh of other people still comprise the entirety of temptation, and that that Satan must use forms in order to foment his waywardness in us.
Perhaps most importantly, it is important that we always remember Paul's declaration that "we wrestle not against flesh and blood" (Ephesians 6:12). This does not preclude worldly and fleshly responsibility in matters of temptation. The same Paul often plainly chided people who opposed or compromised the Gospel. "Alexander the coppersmith did me much evil; the Lord reward him according to his works" (II Timothy 4:14). Still, the heart of temptation always begins and is empowered by spiritual influences who seek to hinder our faith and obedience to the Lord Jesus. Failure to understand this leads us either to misguided spiritual battle, or to no fighting of the good fight of faith at all. We must identify our enemies if we are to successfully overcome them through the power of the Spirit of God and the Word of God. Just as importantly, we must not view other people as the primary instigators of temptation and spiritual attack upon us. They may be tools of the devil, just as sometimes we ourselves are in the lives of others. However, "flesh and blood" do not begin or empower temptation. Spiritual influences occupy this nefarious office, and successful waging of battle against them requires that we know the nature of our true enemies.
Such truth is cause for much confidence and rejoicing because "greater is He that is in you than he that is in the world" (I John 4:4). Indeed, if we perceive flesh and blood to be our primary challenge, our tendency will be to fight with flesh and blood weapons. Conversely, if we rightly understand the spiritual challenges we face, we will more likely "put on the whole armor of God, that ye may be able to stand against the wiles of the devil" (Ephesians 6:11). We will also be far less likely to fall into bitterness regarding the human agencies through which the devil tempts and attacks us. They are responsible to God, of course, and will answer to Him for their participation with Satan and his minions (Luke 17:1). Still, we recognize that the pawns are not the power, nor are they the originators of the challenges we face. Growing understanding of this truth engages us in the spiritual battle fought not for victory, but from victory. The Lord Jesus Christ is risen from the dead, and has overcome every enemy. Through Him, God "always... causeth us to triumph in Christ" (II Corinthians 2:14). And through Him, we fight our real enemies and thus are liberated to far more bestow grace and mercy upon the human pawns with whom we do not wrestle, but to whom we bestow the love of Christ.
"For though we walk the flesh, we do not war after the flesh. For the weapons of our warfare are not carnal, but mighty through God to the pulling down of strongholds." (II Corinthians 10:3-4)
Monday: the weapons of our warfare