The devil wants your mind. He would rather get you to think wrong than to do wrong. The thought is the father of the deed. If he can get you to think about a wrong thing, repeat it enough times to believe it, and you will end up believing it. 

Romans 8:7, ''Because the carnal mind is enmity against God; for it is not subject to the law of God, neither indeed can be.''

Romans 12:2 says, Do not be conformed to this world...but be transformed (changed) by the [entire] renewal of your mind... (AMPC).  

We are in a spiritual battle, and we must fight with spiritual weapons. God has given us His Word as one of the main Arsenal of war and of defense against the lies of the enemy.