For those who are converted the question is not whether they should or should not engage in spiritual warfare but rather how they should fight the battle. When a person receives God's spirit they immediately, whether they like it or not, enter into a war-like struggle against two major spiritual opponents. They will have to fight these two powerful enemies every day of their life until they die.
The first major spiritual opponent Christians face is himself or herself. They must battle their own human nature with its self-defeating, sinful, and destructive tendencies. The apostle Paul referred to this constant warfare of the mind Christians face in his book to the Romans.
For I delight in the law of God according to the inward man; But I see another law within my own members, WARRING against the law of my mind, and leading me captive to the law of sin that is within my own members (Romans 8:22 - 23).
The second lifelong spiritual opponent true believers must face is the devil and the world he has deceived into thinking and acting like as does (Ephesians 2:2, 1John 5:19, Revelation 12:9, etc.). Jesus, in his prayer to the Father before his crucifixion, reveals the ultimate attitude of those who do not believe God against those who do.
I have given them (the disciples and all Christians by extension) Your words, and the world has HATED them . . . (John 17:14)
Paul tells us, clearly, that believers do not fight a carnal warfare common in the world but rather one that is on a spiritual plane.
Because we are not wrestling against flesh and blood, but against principalities and against powers, against the world rulers of the darkness of this age, against the spiritual power of wickedness in high places (Ephesians 6:12).
Resisting evil
How do we resist not only the pulls of our human nature but also Satan and the spiritual powers he has deceived? Paul's analogy to warfare in Ephesians 6 explains various tools God gives us for our battles. First, more generally, Paul writes that to do well at resisting self and Satan, we need to, ‘take up the whole armor of God' (Ephesians 6:11). He then discusses a list of "equipment" that is purely defensive in nature.
The only offensive spiritual weapon in a Christian's arsenal, according to Paul, is "the sword of the Spirit, which is the Word of God" (verse 17). A believer who takes their calling seriously should know the Bible well enough so that he or she can use it to fight false ideas and arguments that would lead them to sin and away from the Father. Being able to know and quote scriptures, however, is not enough. According to Jesus, we must also have one other thing in order to be effective fighters.
The first temptation the Bible lists that Satan threw at Jesus was for him to prove he was God's son by alleviating his hunger (Luke 4:3, Matthew 4:3). Jesus' reply was to quote Deuteronomy 8:3 that man is to derive his life not just from physical food but also from SPIRITUAL food - the words of God (verse 4).
The devil's third temptation, however, was very deceptive and subtle. He transported Jesus to the top edge of Jerusalem's temple then challenged him to prove he was God's son by jumping off! After all, God's adversary stated, does not the Bible say that God has angels watching over Jesus who would, if he jumped, SAVE HIM for any harm? This temptation was incredibly subtle since Satan perfectly quoted to Jesus God's words in Psalm 91:11 - 12.
What was Jesus' response? He did not refute the quote itself because it was accurate. What Jesus refuted was the wrong interpretation and sinful application of the verses by stating, "You shall not tempt the Lord your God" (Luke 4:12).
To be sure, knowing what the Bible says is important in our daily spiritual warfare. However, it is not enough. The devil and his demons ALSO know what the Bible says and could probably quote it better than most of us. What is needed (which the powers of darkness do not do) is how to properly interpret and apply the scriptures to life and the trials and troubles that come our way.
How do we gain what is required in our spiritual warfare in order to not only fight effectively but also to be ultimately victorious? The book of James gives us the answer. We are to submit to God and resist Satan. We are also commanded to 'Draw near to God (through prayer, fasting, study, fellowshipping with others who are converted, etc.), and He will draw near to you' (James 4:8).