The Battle is in the Mind
Matthew 21:21 "Jesus answered and said unto them, Verily I say unto you, If ye have faith, and doubt not, ye shall not only do this which is done to the fig tree, but also if ye shall say unto this mountain, Be thou removed, and be thou cast into the sea; it shall be done."
Jesus makes it very clear that we must have faith and doubt not. Every believer has faith, but most believers also have an abundance of doubt that negates their faith. It is when we purify our faith and only believe, that victory comes.
Every Christian is at war. There is a perpetual struggle against Satan and his kingdom where there are no "leaves" or "discharges." Our enemy goes about as a roaring lion, seeking whom he may devour (1 Peter 5:8). Those who resist the devil will see him flee (James 4:7). The only ones whom he devours are those who don't actively fight against him.
The mind is the battlefield where thoughts and reasonings that are contrary to God's Word, need to be captured and submitted to Christ, our Commander.
Just as enemy soldiers are captured in war, so rebel thoughts must be taken captive and made to submit to Christ. Our battle against the devil takes place right between our ears. The spiritual weapons given to us are designed for the express purpose of taking EVERY thought captive and making them obedient to Christ. Keeping our minds completely stayed upon the Lord is an obtainable goal.
These weapons of ours are for the casting down of two things: Imaginations; and every high thing that exalts itself against the knowledge of God. Both of these areas are dealing with the mind. Focus on the good in every area of your life. Failure to recognize God's blessings in every day living will cause care and anxiety. Recognizing God's hand in even the smallest thing will cause peace and keep our hearts and minds following hard after the Lord.