Do not be overcome by evil, but overcome evil with good. (ESV)
We live in a world consumed by evil. In the Garden of Eden mankind was warned of the death that would come to them if they sinned against God's command. Man's sin brought death, not only to the first couple, but to all of humanity for all generations. Spiritual death is the root cause of all the trouble that we encounter in the world.
This instruction that Paul gives to us in Romans 12:21 is for those who have placed their faith in Jesus Christ. We know that our faith in Jesus grants us forgiveness of sin and the hope of eternal life. In the meantime we still have to deal with the effects of evil that are all around us. As Christians, how we deal with evil is very important to how effective we communicate the Gospel.
We are not to be overcome by evil. A life lived for Christ is going to be a life in which we will suffer persecution. Those who are evil are ultimately doing the bidding of the enemy. If we are not strong in our walk with the Lord the attacks of the enemy will be more than we can bear. Having a proper understanding of the power of evil helps us to prepare accordingly.
So, how do we prepare ourselves to avoid being overcome by evil? We plan on overcoming evil with good. When the enemy attacks us through a person who brings persecution we choose not to retaliate. We offer love when they offer hate. We do not hang on to bitterness which only destroys us, but rather we demonstrate the same love of Jesus that we ourselves have received.
Evil is real. Evil is headed our way in the form of persecution by people who do not love the Lord. Stay close to the Lord. Plan beforehand to offer forgiveness and love to those who do not treat you with anything but hatred and contempt.