Life transformation is rooted in mind transformation.
Where your mind goes, your actions follow. Whatever you think about will dictate your behavior, your attitude, and (ultimately) your direction in life. Your thoughts are sort of like a rudder—they steer the rest of your body.
Considering the influence of your mind on your life, it's no surprise Paul emphasizes that in order for our lives to be changed by Christ our thoughts must first be changed by Him: Do not conform any longer to the pattern of this world, but be transformed by the renewing of your mind. Then you will be able to test and approve what God's will is—his good, pleasing and perfect will (Romans 12:2). How are we to be transformed? By renewing (changing, refocusing) our minds so that they reflect the mind of Christ.
If our minds are so important, then it's quite understandable that the devil would fight to gain a foothold in our thoughts. Paul writes, in 2 Corinthians 4:4, The god of this age has blinded the minds of unbelievers, so that they cannot see the light of the gospel of the glory of Christ, who is the image of God.
What better place for the devil to attack people than in their minds? As thunder follows lighting, so actions follow thoughts. And if Satan can turn your thoughts against God, your actions will assuredly follow.
Therefore, it's imperative that you set your minds on things above, not on earthly things (Colossians 3:2). And when you focus your mind on God, it's only a matter of time before your actions, your passions, your attitudes—your whole life—become focused on God.