Before we can answer this question properly, it is imperative that we first understand who Jesus Christ is, as well as the Divine Mission which God the Father gave to his "only begotten Son". By understanding the nature of the Kingdom of God and its relationship to the demonic kingdom, we begin to understand the foundational principles of spiritual warfare, and it is only in this context that we can truly define the term "spiritual warfare" and learn how these daily battles must be fought by the faithful of Christ's Church.
The Kingdom of God
The primary purpose of Jesus Christ on the Earth was to inaugurate the Kingdom of God. The Sacred Scriptures, in Luke 4:43, tell us: "But [Jesus] said to them, 'To the other towns also I must proclaim the good news of the kingdom of God, because for this purpose I have been sent.'" It is quite clear here that the Messiah understood His mission with clarity and Divine resolve. He was to announce the arrival of His Father's kingdom to the whole world.
We are given again more insight into this mysterious kingdom in Luke 17:20-21, where it reads: "Asked by the Pharisees when the kingdom of God would come, [Jesus] said in reply, 'The coming of the kingdom of God cannot be observed, and no one will announce, ‘Look, here it is,’ or, ‘There it is.’ For behold, the kingdom of God is among you.'” Christ Himself, as this dialogue suggests, brings the Kingdom of God to us.
In his book Jesus of Nazareth, Pope Benedict wrote that "the Kingdom is not a thing, it is not a geographical dominion like worldly kingdoms. It is a person; it is he. On this interpretation, the term ‘Kingdom of God’ is itself a veiled Christology. By the way in which he speaks of the Kingdom of God, Jesus leads men to realize the overwhelming fact that in him God himself is present among them, that he is God’s presence.” In other words, the Kingdom of God comes to us through the direct Presence of Jesus Christ, the Pope mirroring the Gospel of Luke's message above.
Furthermore, we discover that, when this kingdom comes, it has a profound spiritual impact on those who are touched by it. As the writer of Mark 2:12 describes of those who personally witnessed the authority and miraculous power of Jesus's ministry: "They were all astounded and glorified God, saying, 'We have never seen anything like this.'"
What is the Kingdom of God?
So, what is the Kingdom of God? It is obvious from the Gospels that it is not an earthly dominion, one whose boundaries can be found on a map. Christ hints in John 18:36: “My kingdom does not belong to this world. If my kingdom did belong to this world, my attendants [would] be fighting to keep me from being handed over to the Jews. But as it is, my kingdom is not here.”
Again, Pope Benedict helps us understand this principle even more. "Those who pray for the coming of the Kingdom of God pray without any doubt for the Kingdom of God that they contain in themselves, and they pray that this Kingdom might bear fruit and attain its fullness. For every holy man it is God who reigns … So if we want God to reign in us, then sin must not be allowed in any way to reign in our mortal body … Then let God stroll at leisure in us as in a spiritual paradise and rule in us alone with his Christ."
The Kingdom of God, the Pope instructs, comes to dwell within us! It re-makes us so that "whoever is in Christ is a new creation: the old things have passed away; behold, new things have come" (2 Corinthians 5:17). We are new creations when Christ comes to rest within us, when He has established His Kingdom in our hearts. This is when He begins to fashion us in the image of His likeness, that we might bear the light and love of Christ to the whole world.
How are Members of the Kingdom Identified?
In Matthew 13:37-43, Jesus is approached by His disciples, who have just listened to Him preach the famous Parable of the Sower, in which He describes a man sowing seed in a field. Some of the seed is eaten by birds before it can take root in the ground, while other seed prematurely grows without a good foundation as they fall upon hard, rocky ground, and this causes them to wither and die when the sun comes up as they cannot be nourished by water. A third kind of seed also falls along the field, and the "thorns of life," so to speak, spring up around it as it blooms, choking the plant and depriving it of vital nutrients, which can ultimately kill it. And finally, the fourth and final kind of seed, Christ describes, falls on good soil, is watered, and springs up to a full and abundant life.
The disciples, who are confused by this parable, ask their Teacher to explain it. Christ responds: “He who sows the good seed is the Son of Man. Now the field is the world. And the good seeds are the sons of the Kingdom. But the weeds are the sons of wickedness. So the enemy who sowed them is the devil. And truly, the harvest is the consummation of the age; while the reapers are the angels. Therefore, just as weeds are gathered up and burned with fire, so shall it be at the consummation of the age. The Son of Man shall send out his angels, and they shall gather from his Kingdom all who lead astray and those who work iniquity. And he shall cast them into the furnace of fire….”
Here we have a clear line in the sand, drawn by Christ Himself, which helps us identify those who are members of the mystical Kingdom of God based on their good works. Christians are the "good seeds" which grow into healthy, fruitful plants, and it is by the very fruits produced during their season here on the Earth (their good works, in other words) which Christ will specifically use to judge them at the end of all things.
But in this very same passage there is also, troublingly, another kingdom mentioned.
The Kingdom of Darkness
Just before His crucifixion and Passion, Jesus prayed to the Father, "I do not pray for the world but for the ones you have given me, because they are yours, and everything of mine is yours and everything of yours is mine, and I have been glorified in them" (John 17:9-10). Now why would Christ not pray for the world?
Romans 5:12-14 helps us to understand: "Therefore, just as through one person sin entered the world, and through sin, death, and thus death came to all, inasmuch as all sinned—for up to the time of the law, sin was in the world, though sin is not accounted when there is no law. But death reigned from Adam to Moses, even over those who did not sin after the pattern of the trespass of Adam, who is the type of the one who was to come."
The world is full of sin and death. Christ does not pray for it because it is evil, filled to the brim with unrepentant sinners. And it is a world that is made in the likeness of its god. In 2 Corinthians 4:4, the passage reads: “…the god of this age has blinded the minds of the unbelievers, so that they may not see the light of the gospel of the glory of Christ, who is the image of God.”
And who is this god? Who is the ruler of this world full of death and evil?
Christ explains who. "The father from whom you are is the devil, and the desires of your father it is your will to do. He was a murderer from the beginning and has not stood in the truth because there is no truth in him. When he tells a lie, he speaks from his very nature, for he is a liar and the father of lies” (John 8: 44).
The Devil is the ruler of this world, the god of this age, the voice which seduces us to wickedness and disobedience. In the Gospel of Luke 4:5-6, it reads: “Then [the Devil] took him up and showed him all the kingdoms of the world in a single instant. The devil said to him, ‘I shall give to you all this power and their glory; for it has been handed over to me, and I may give it to whomever I wish.’”
The Church teaches us that the Devil is a fallen angel named Satan. He rules a kingdom of darkness that possesses a certain amount of power and influence over the Earth. In Catechism 391 & 394, it reads: "Behind the disobedient choice of our first parents lurks a seductive voice, opposed to God, which makes them fall into death out of envy… called 'Satan' or the 'Devil'… Scripture witnesses to [the Devil’s] disastrous influence…who would even try to divert Jesus from the mission received from his Father. 'The reason the Son of God appeared was to destroy the works of the devil.' In its consequences the gravest of these works was the mendacious seduction that led man to disobey God."
The Devil, as the Catechism shows us, is diametrically opposed to Christ's mission on the Earth, because Christ's mission on the Earth, His purpose in bringing to it the Kingdom of God, is to "destroy the works of the Devil"--in other words, all that the Evil One has worked towards throughout history, starting with his rebellion in Heaven and his seduction of man, which has led the whole world astray. Christ has come to destroy Satan's kingdom. "The huge dragon, the ancient serpent, who is called the Devil and Satan, who deceived the whole world, was thrown down to earth, and its angels were thrown down with it" (Revelation 12:9). Satan's kingdom of darkness, which consists of his fellow rebel angels, called demons, and his human followers on the Earth, whom Christ refers to as the "sons of wickedness," seek to oppose in every way the Kingdom of God.
But why does this cosmic battle of good and evil involve us?
You see, sin and death entered the world through Adam and Eve's first sin, and all of mankind has been enslaved by the kingdom of darkness because of their willful decision to follow the "god" and ruler of an unrepentant world. Jesus once said: “Amen, amen, I say to you, everyone who commits sin is a slave of sin" (John 8:34). What does Christ mean by this, that the human condition is now one of slavery?
In Romans 6:17, it reads: “But thanks be to God that, although you were once slaves of sin, you have become obedient from the heart to the pattern of teaching to which you were entrusted.”
The writer of Romans, St. Paul, explains that those who are members of the Kingdom of God have been set free from slavery and bondage to sin, to death, and to the power of Satan and his demonic kingdom! The work of Christ, the work of the Kingdom of God, then, is to destroy the works of the Devil, in particular by setting those who have been enslaved to the Evil One free from their condition.
Christ said, "But if it is by the Spirit of God that I drive out demons, then the kingdom of God has come upon you" (Mathew 12:28). Notice here that Jesus tells the crowd listening to Him that it is, in fact, the authority Christ has to cast out demons which demonstrates the power of the Kingdom of God over the kingdom of darkness.
This is the ultimate purpose of the Kingdom of God: to drive out demons, to break the power of the Evil One over humanity, to convert human souls to the Truth and to the worship of the One True God, through his Christ, for the Kingdom of God is greater than the kingdom of the one who is in the world.
Further Proof: The Exodus of God’s People
For further proof of this, we turn to the Book of Exodus in the Old Testament and the remarkable story of God's deliverance of the Hebrews from Egyptian slavery.
As the story tells us, the Israelites, who are God's people, have been enslaved by a tyrannical Egyptian king, the Pharaoh. But God sends a deliverer named Moses to set them free. He gives Moses great power and authority over the Egyptians, and the Prophet makes a fool of them time and time again through God's miraculous wonders. Biblical scholars have noted that the plagues God sends upon Egypt through Moses seem to correlate with the different pagan gods of the Egyptian pantheon, and while the magicians, who gain their power from these so-called "gods" (who are, in truth, demons in disguise), are able to copy some of Moses's initial miracles, they quickly find themselves outmatched, as God demonstrates quite handily that he and he alone is the Creator of the Universe. This is another interesting nod at the gap in power between the Kingdom of God and the demonic kingdom. Satan can act upon the world in a limited fashion, but in the end, he is still nothing more than a creature, and cannot even approach the power of God.
As the Ten Plagues continue to fall upon the Egyptian nation as punishment for the Pharaoh's disobedience to God and for the mistreatment of God's people, Moses is commanded to have each Hebrew family slay a lamb and mark the lintel of their doorway with its blood. Through this sacrifice, the people are spared from God's wrath that very night when the angel of death passes over each of their homes. But the Egyptians remain unprotected when this final plague strikes, and the firstborn of every Egyptian household is struck dead as a result, including the Pharaoh's son.
In his grief, shame, and humiliating defeat, the Pharaoh frees the Hebrews at last.
But a few days after this, the Pharaoh's heart is again hardened. He gathers his armies and pursues after the Hebrews, trapping them along the edge of the Red Sea. God rises up and defends his people, destroys Pharaoh's armies, and the Hebrews escape forever to enter the land promised to their forefather Abraham centuries before.
Typology for Christ and the Spiritual Battle
This story is very similar to the current condition of the world in the grip of the cosmic spiritual battle between the Kingdom of God and the demonic kingdom. In fact, it is actually a “type” (or symbol) pre-figuring the victory of Christ.
The entire human race, who were made in the image of God at their creation (Genesis 1:27), are God's people, symbolized by the Hebrews in the Exodus story. But they have been enslaved by an oppressive tyrant, who causes them only great suffering. This tyrant is none other than Satan, the ruler of the kingdom of darkness. But God sends a deliverer, who is the Messiah, Jesus Christ. He performs many miracles, including demonic exorcisms, to show the world His authority over Satan, and finally, at the end of His ministry, just before Jesus entered into His Passion and Crucifixion, He demonstrates to the entire world His power over death itself, the most horrific of Satan's works, for when the Devil tempted Adam and Eve and succeeded in convincing them to sin, death was the ultimate result.
Then Christ becomes the sacrifice which protects us from the Second Death, which is eternal damnation (Revelation 21:8). John the Baptist identifies Jesus as such when he declares: "Behold the Lamb of God, who takes away the sins of the world" (John 1:29). Through His Crucifixion and death at the hands of the Romans, Jesus becomes the sacrificial Lamb of God, much like those lambs which were slaughtered by the Hebrews. It is Jesus's Precious Blood, shed for our sins and for our freedom from bondage to the Evil One, which saves us from eternal death, or damnation. The "angel of death," so to speak, can no longer touch us. We are freed by this redemptive act, and are no longer enslaved to sin and death. We have the chance to attain everlasting life and freedom to be with and know God in an intimate relationship again, as we once did long ago in the Garden of Eden.
But, like the Pharaoh in the book of Exodus, who pursued the Hebrews after they fled his kingdom and gained their freedom, Satan has also turned back after us, despite his grand defeat and humiliation at Christ's Resurrection. The Devil seeks to re-enslave us again. And he can!—but only if we choose to sin and place his yoke about our own necks again with our own free will, as some of the faithless Hebrews proposed to do when they saw Pharaoh's huge armies descending upon them, and they forgot what God had done for them.
Christians, despite their choice to pursue a life of righteousness and obedience to God, despite even their Baptism, can turn away from that choice at any time and become enslaved to sin and Satan again. This is because of their free will. As the writer of James declares: “You adulterous people! Do you not know that friendship with the world is enmity with God? Therefore whoever wishes to be a friend of the world makes himself an enemy of God” (James 4:4). Notice that he is referring to baptized Christians here. He is saying that they can, in fact, become enemies of God again by befriending the world through their actions.
And as St. Paul wrote to the Christians in Galatia: “Now the works of the flesh are obvious: immorality, impurity, licentiousness, idolatry, sorcery, hatreds, rivalry, jealousy, outbursts of fury, acts of selfishness, dissensions, factions, occasions of envy, drinking bouts, orgies, and the like. I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God” (Galatians 5:19-21). Again, St. Paul is warning these Christians, who are already baptized, that they will not inherit the kingdom of God if they keep sinning, thereby enslaving themselves to Satan and the demonic kingdom.
Spiritual Warfare Defined
With this background in mind, that of a Greater Kingdom of Christ expelling the demonic forces of the lesser kingdom of Satan from the lives of men, we now come to a proper definition of spiritual warfare.
You, like every other person in history, have been born in the midst of a battle over your soul. Every day, whether you realize it or not, you are locked in a struggle to reject sin, death, and Satan in order that you might be able to receive the message of salvation preached by Christ and His Apostles, who are now, through apostolic succession up and into the modern era, the college of bishops under the leadership of the Pope (in other words, the Catholic Church).