When you submit your life to Jesus Christ as your Lord and Savior, God delivers you from the domain of darkness and transfers you to the Kingdom of His Son.
Before Jesus saved you from sin and death, you were an enemy of God and a slave in the domain of darkness. You were under the power of the evil one (1 John 5:14).
Now that you belong to Christ, you are a son/daughter in the family of God and a citizen of His Kingdom.
That also means you have a new enemy whose sole purpose is to kill, steal, and destroy everything. The devil, the Enemy of God, has waged war against God’s people. His cruel hatred is now directed at you.
That is why God commands us to stand firm and put on the armor of God. Read Ephesians 6:10-17:
6:10 Finally, be strengthened in the Lord and in the strength of his power. 6:11 Clothe yourselves with the full armor of God, so that you will be able to stand against the schemes of the devil. 6:12 For our struggle is not against flesh and blood, but against the rulers, against the powers, against the world rulers of this darkness, against the spiritual forces of evil in the heavens. 6:13 For this reason, take up the full armor of God so that you may be able to stand your ground on the evil day, and having done everything, to stand. 6:14 Stand firm therefore, by fastening the belt of truth around your waist, by putting on the breastplate of righteousness, 6:15 by fitting your feet with the preparation that comes from the good news of peace, 6:16 and in all of this, by taking up the shield of faith with which you can extinguish all the flaming arrows of the evil one. 6:17 And take the helmet of salvation and the sword of the Spirit (which is the word of God).
You are at war whether you like it or not.
“Be sober and alert. Your enemy the devil, like a roaring lion, is on the prowl looking for someone to devour.” (1 Peter 5:8)
It is dangerous to ignore the reality that you have an enemy who is always trying to destroy you. Christians can neither fight the evil one in their own strengths nor stand against him on their own righteousness. Jesus Christ in us has decisively destroyed the works of the devil.
“You are from God, little children, and have conquered them, because the one who is in you is greater than the one who is in the world.” (1 John 4:4)
Remain in Christ and let the word of Christ richly dwell in you (John 15:7; Colossians 3:16). Having thus submitted to God, resist the devil, and he will flee from you (James 4:7).
You can overcome evil with good because Jesus has already won the war even though spiritual battles are still raging all around you.