We hear a lot about spiritual warfare, and perhaps we don't understand it very well. It may seem like some mystical conflict between us and satanic powers, full of Book-of-Revelation images of beasts and dragons. On a certain level, it actually is that. But our practical daily "warfare" is more subtle, and hence we may not even be aware of the battles we're losing.
One way to look at it is to see it as a "war of worlds." There are a number of "worlds" in which we live, some of which need to be "baptized" into the Gospel, and others need to be entirely severed from us. Some examples may be the business and employment world, the world of leisure and recreation, the world of information and entertainment, and the dark world of satanic evil.
Our main world in which we live ought to be the world of God, of prayer and truth and love. This world should influence other worlds we need to have contact with, and must shield us from the evil world. It might seem simple enough for us to embrace whatever is good in them, and reject whatever is evil, but unfortuately it isn't. Why not?
Each "world" has its own logic and rationale, and when you accept its presuppositions, mentality, and psychological "environment," then it will begin to make sense to you, and you will be comfortable living in it. For example, if a certain amount of lying and cheating and "doctoring the books" is part and parcel of the business world, and if it seems to work for you, you may find yourself thinking that it is OK. Or if you immerse yourself in the mindset and perspectives of what you find in magazines, TV, and movies, its own peculiar logic might eventually make sense to you, because if you accept faulty premises, you will accept faulty conclusions as well. If you engage in such things as impure fantasies or desires (which can be called "deceitful lusts," Eph. 4:22), you will eventually find that even what is contained in that evil world will make sense to you, because you have placed yourself within its psychological and spiritual framework. Evil thoughts make perfect sense within an evil world, and you will start asking yourself why anyone would think it wrong.
The spiritual warfare consists in not allowing ourselves, in mind and heart, to come near enough to any world that is not God's world (or that cannot be easily integrated into God's world), so that it could draw us in. Once we adopt the mentality of the other worlds, we have lost our strength to fight. The evil world encroaches upon us, much like a virus that approaches a healthy cell: a virus injects its own genetic information into the healthy cell and destroys it from within, reproducing itself as a parasite within the cell.
We must not allow evil to do this to us. Where possible, we must introduce the mind of Christ into the mentality of a world in which we must still function, like the world of commerce and government. When dealing with a world that is nothing but evil (the arena of satanic temptations and deceptions), we must sever all ties to it absolutely. For as soon as we start thinking that there might be something OK about what the devil offers us, we have entered his world and will find it very difficult to escape it without sin. We must constantly turn to the Lord, to his world, seeking first his kingdom and righteousness--the teachings of the Bible and the Church will make clear what constitutes God's world--and then all evil will be unmasked as such, so that we are not deceived and drawn into a world that will be our downfall.
So you see, spiritual warfare is not dueling Darth Vader with light sabers. The arena is our own heart and mind, and the places we live and work. Our weapons are truth and purity of heart, the Scriptures and the Sacraments. Choose the world in which you want to live. "Resist the devil and he will flee; draw near to God and He will draw near to you" (James 4:7-8).