“For though we walk in the flesh, we do not war after the flesh: for the weapons of our warfare are not carnal, but mighty through God to the pulling down of strong holds, casting down imaginations, and every high thing that exalteth itself against the knowledge of God, bringing into captivity every thought to the obedience of Christ.”  2 Corinthians 10:3-5

The battleground is in the mind.  We must measure every thought against the word of God.  If it’s not aligned with His will, we pull it down by saying no to it and replacing it with His word.  If the adversary can capture our thoughts and imaginations he can gain access to our lives.  If there is a place where he has already deceived you, then repent, and come back in line with the word.