C.S. Lewis stated "The greatest trick the Devil ever pulled was convincing the world he didn't exist." I point this out because though the devil is defeated in the finished work of Jesus Christ on the cross, we are not to be naive about the fact that he is in the world, just as we are in the world, and he comes "only to steal and kill and destroy;" (John 10:10).
Be of sober spirit, be on the alert Your adversary, the devil, prowls around like a roaring lion, seeking someone to devour. - 1 Pet. 5:8
It's important to realize what Peter is saying here, Satan is after you, and to not realize that, not to be on alert makes you easy prey. The good news is, "greater is He who is in you than he who is in the world" (1 John 4:4), and Satan will never pluck you from Jesus hand (see John 10:28, Rom. 8:38-39). However this is not to say he can't effect your daily walk with Christ. If Satan could have no effect at all, then why the many warnings in God's word? God's word has recorded Satan inflicting disease (Luke 13:10-17, Matt. 17:14-18), entering into people (Luke 8:30, 22:3,) and even influencing peoples very thoughts. (Matt. 16:21-23[below]).
From that time Jesus began to show His disciples that He must go to Jerusalem, and suffer many things from the elders and chief priests and scribes, and be killed, and be raised up on the third day. Peter took Him aside and began to rebuke Him, saying, "God forbid it, Lord! This shall never happen to You." But He turned and said to Peter, "Get behind Me, Satan! You are a stumbling block to Me; for you are not setting your mind on God's interests, but man's." - Matt. 16:21-23
Peter wasn't even rebuked here, Satan was. Even though the very thing Jesus was rebuking was delivered by Peter's mouth. Let me ask, do you think Peter knew those thoughts were from Satan? Most certainly he did after the rebuke, but before? I think not. For if Peter had known, he wouldn't of said those things. Also if you read the verses immediately prior to this (Matt. 16:13-20), Jesus just got done commending Peter for confessing Jesus as the Christ, the Son of the living God. So in one instance Peter understands and confesses Jesus as God, and the Messiah, and in the next instance he rebukes Jesus telling Him that things in which He just prophesied shall never happen, he even adds "God forbid it, Lord!"
How often do we take thoughts implanted by the enemy and claim them as our own? I had an interesting discovery where I would take my doubts or thoughts I was struggling with and write them on paper. I was amazed at how outlandish and even ridiculous some of those thoughts were once I brought them to light by writing them down. I found this also to be a good tool in "taking every thought captive to the obedience of Christ" as instructed in 2 Cor. 10:5 when fighting "every lofty thing raised up against the knowledge of God". Jesus gave us an excellent example of how to fight the father of lies (John 8:44) when He was attacked by Satan and tempted with doubts. Matt. 4:1-11 record these events, and notice in the first two temptations, Satan started out saying to Jesus "If You are..." Tempting Jesus to doubt who He was and to prove who He was by doing some miraculous thing. Of course these temptations failed miserably against our Lord. Nonetheless the temptation to doubt was evident. This was no new trick. Man's original sin started out with a temptation to doubt from Satan. Genesis 3:1 has Satan talking to Eve saying "Indeed, has God said..."
What does this mean for the doubter? 2 Corinthians 5:7 tells us that we walk by faith and not by sight, so if Satan who's name is defined in Hebrew as "adversary" wants to attack our daily walk with God, than he would attack our faith in attempting to make us walk by sight. Peter by faith walked on water, but when he switched to walking by sight he sank, (see Matt. 14:29-31). As discussed in previous chapters, doubt and faith can co-exist, but if Satan can get us hung up on the doubts, we will easily quit walking by faith.
We have two main defenses against these attacks, one is passive, the other is active. Passive is the security we already have in Christ which has God doing the fighting for us and on our behalf (Rom. 8:33-34). This is the fight that we know by God's word is done, and Jesus won. This is where the accuser of the brethren is thrown down by the power of Christ (Rev. 12:10, see also Zech. 3:1). This is where Jesus has fully condemned all of our sin on the cross, thus saving us entirely by His doing. (Col. 2:13-14, Eph. 2:8-9), This is where nobody, including Satan can take us from Christ's hand. (John 10:28, Rom 8:38-39).
The other aspect is active. Mainly, this is done by trusting God, trusting in His work (some of what is mentioned in the above paragraph), standing strong in the strength of His might (Eph. 6:10). In the armor of God mentioned in Ephesians 6:10-16 it is by taking up the shield of faith we extinguish all the flaming arrows of the evil one. Doubt is one of those arrows. So how do we take up the shield of faith? The same way Jesus did in Matt. 4:1-11, by taking God at His Word. This is our resistance, and God has promised that if we resist the devil, he must flee (Jam. 4:7).
It is easy to credit a problem such as doubt all to Satan. Often well meaning colleagues will do just this when assessing our problem, which leaves us the doubter feeling even more helpless and increases the intensity of our fear as we already secretly worried this was the case. Often this tempts us to take desperate measures that are irrational because we think If only I could get these demons cast out, then my doubts would be gone.
I wondered if I was demon possessed, when emotions reach a level of intolerability you wonder anything. This is a common fear by those who can't escape their doubts. A mentor of mine stated she had never met a doubter who has been genuinely demon possessed, neither have I. Therefore we should be careful assessing the origins of our doubts. Satan is a foe that God felt worthy of mentioning, and as we've seen above the devil does use doubt as a tactic, but to give him to much credit and not consider multiple sources of our doubts can be counter-productive to our victory over this struggle.