I recently read a book on spiritual warfare, written by someone who used to be a satanist, but who now has happily returned to the Church of her birth and baptism, the holy Catholic Church. (Deborah Lipsky, A Message of Hope: Confessions of an Ex-Satanist.) Perhaps not everything in this book is worth recommending (and it would have benefited from a competent editor and proofreader), but there’s one main point I’d like to reflect upon, which probably applies to just about all of us. It’s about avoiding negative or otherwise spiritually harmful thoughts.
At the outset I should make a distinction. Not all “negative” thoughts are bad thoughts. For example, I know someone who says things like this, if you happen to bring up the subjects of sin, repentance, the manifest evils in the world, etc: “Don’t say those things to me. They are negative thoughts. Negative thoughts cause stress, and I choose to live without stress, thinking only positive thoughts.” To have this attitude is to live in La-La Land and to be disconnected to reality. But there still are a lot of thoughts that people commonly indulge in that really ought to be avoided, because they are open invitations to demonic manipulations of our souls.
Since demons are spirits, they have no material element in their beings. They are all spiritual energy, and not matter. Our own thought processes, 
though they use the material brain as an instrument, still belong to the immaterial soul and so are primarily spiritual in nature. Now even though the demons can’t literally read our every thought, they can sense the spiritual “energy” that our thought processes produce, and if this is angry, hateful, lustful, malicious, etc, they will recognize the evil “scent” and will hurry to it like sharks after blood.
The demons “feed” on anything that is wicked or unclean, and their appetites are insatiable. (A text in one of the Byzantine Lenten Offices repeats, “O compassionate Lord… let me not become the possession and food of the enemy…”) So when the demons are attracted to a soul emitting all this foul spiritual energy that they like to feed on, they will try to increase it, to suggest certain trains of thought that will lead to deeper sin. They will try to influence us to form habits of thought that are dark, negative, critical, vengeful, impure, etc. If they succeed in this, they will have frequent feeding frenzies on our souls, and we will become more and more mired in the ruts of our harmful thoughts, which, even though they may begin as directed toward people we resent or hate, become ultimately self-destructive, and this is what the evil spirits are after.
Even without much interference from the devils and their spiritual powers, we can get into mental ruts through our own lack of vigilance over our thoughts. The brain itself tends to form pathways for neurons to connect in a certain way when we repeatedly speak or act in some way that gives us some form of pleasure (even the perverse pleasure of holding a grudge or indulging in something sinful). Eventually it “urges” us to keep repeating the behavior that gives pleasure, and this is one way that addictions are formed. The demons, however, won’t sit idly by, waiting for all this to happen. They will actively try to hasten this process through temptations and seductions of various sorts. Their goal isn’t merely to get us stuck in bad habits, though. That is only the means to the end of getting us to fall into despair, turn away from God, and, if all goes well (from their perspective), to get us finally to choose suicide as the only way out of the maddening torments that have taken over our interior lives, over which we have lost all control.
So what can we do to short-circuit this process and refuse to give the demons something to feed on, or worse, to actually commandeer our souls if we leave ourselves wide open enough? The short answer is: Stop having bad thoughts! That’s easier said than done, however, so we have to employ the means the Church has given us for victory in the spiritual warfare.
St Paul says we have to “put on the armor of God” (see Eph. 6:10-18). This consists mainly of truth (since the suggestions and promises of demons are lies), righteousness, faith, the word of God, and prayer. Use holy water, wear blessed medals, a crucifix, a scapular, etc. (I wear, night and day, several medals, a scapular, and a crucifix containing several precious relics, including one of my dear St Gemma, who is powerful against devils. So what if I sound like jingle bells when I walk. I give the demons fair warning to get out of the way!) We should also invoke St Michael and the holy angels, especially our guardian angels, whose mission it is to protect us from evil, but they will have to stand aside if we use our free will to choose what is sinful. (They cannot manipulate our freedom, as God Himself refuses to do, but if we invoke them, they will help us.) Especially we ought to entrust ourselves to the Queen of Angels, the Mother of God who has been given the power to crush the head of the ancient serpent, her enemy and ours. We have to be 100% on the side of Christ and his truth and goodness and love, for every percentage point less than 100 makes a little opening for the devils to latch on to the dark energy of our sinful thoughts.
We ought also, as far as possible, to avoid the company of those who are constantly creating spiritually polluted environments by the angry, crude, irreverent, or otherwise bad things they say. We don’t want all that junk floating around in our memory banks. The demons are good at reminding us of stuff we’d rather not remember, things we have seen and heard that can be a seedbed for sin. So for God’s sake (and for yours), don’t look at any sort of pornographic or even indecent stuff! This will be burned into your brain’s “hard drive” and it cannot be erased (except by a miraculous divine intervention), even though the sin can be forgiven. You will at some point be reminded of it by the enemy of your salvation, at the most inopportune of moments, like your times of prayer. There are certain “triggers” that can re-connect you to the memory of past sins, and the evil spirits are always looking for ways to get you to relapse.
Along with cleaning out the spiritual junk of bad thoughts, we have to feed our heads with the good stuff. Our thoughts ought to be humble, full of gratitude for God’s gifts and mercies, full of faith and devotion, seeking to say and do what is good. If we are constantly feeding ourselves with what is holy and good, those demonic sharks will not find anything to attract them; in fact, they will be repulsed. Therefore St Paul tells us: “Whatever is true, whatever is honorable, whatever is just, whatever is pure, whatever is lovely, whatever is gracious, if there is any excellence, if there is anything worthy of praise, think about these things” (Phil. 4:8).
A prayer like the Jesus Prayer (“Lord, Jesus Christ, Son of God, have mercy on me”) or some other short prayer that you can often repeat during the day will help cleanse your mind and not leave it idle for a bunch of bad spirits to come and inhabit (see Lk. 11:24-26; note that in the verse immediately preceding this, Jesus says: “He who is not with me is against me”; there is no neutral ground). It really is serious business, and it takes sustained effort and vigilance to keep our minds free of that which can lead us to sin, which pollutes our souls, which makes it harder for the Holy Spirit to continue his work of sanctification, and which opens portals to the prince of darkness.
So, as St Peter reminds us, “Be sober; be watchful. Your adversary the devil prowls around like a roaring lion, seeking someone to devour. Resist him, firm in your faith…” (1 Peter 5:8-9).
Don’t permit yourself, then, to indulge in thoughts that you wouldn’t feel comfortable sharing with Jesus and his Mother. There’s a war going on for your soul, and it won’t be over until you pass the threshold to eternity. Don’t surrender your strength to the enemy by giving him ammunition to use against you. Learn to recognize patterns of thought that tend to lead you to sin, to hate your neighbor, to over-indulge yourself, to grumble or complain—especially against God (the demons like it if we blame God for the hardships or sufferings of our lives, for then we become more like demons ourselves)—or to do anything that is not compatible with the word of God and the holy teachings of the Church. Then cut it off, beg help from Heaven immediately and the angels will come. Replace the bad stuff with good stuff, and thus send the demons packing! They can’t feed on holiness; it is a torment to them.
So resolve to live in the truth, put on the armor of God, be 100% for Christ and 100% against the devil and his works. Pray often, receive the sacraments worthily, keep your eyes fixed on Heaven. Those who allow themselves to be led by every thought that feeds their own passions will end up being “the possession and food of the enemy” for all eternity. Now is the time to “train yourself in Godliness… as it holds promise for the present life and also for the life to come” (1 Tim. 4:7-8). And stop those bad thoughts!
Making All Things New 