2 Corinthians 10:5 (NKJV)
Throughout history there have been spies, people who work for one group or country but try to infiltrate and get into another group or country, to either cause harm or to try to find ways to influence something in the other group or country. The way a spy works is to try and somehow blend into the group or country that they are not from by doing everything they can to seem as though they belong. A successful spy is someone who becomes accepted by the group or country they are working to infiltrate, and once this happens these spies can cause a great deal of damage.
One of the areas we will have our biggest challenges in is in the area of our thoughts. We have to examine and check every thought we have by the Word of God to be sure we catch any wrong or misleading thoughts. Not all bad thoughts are easy to recognize, the devil will try to find a way to make the wrong thoughts blend in and seem normal. The devil will work to try and put thoughts into our mind that seem to be alright, but they lead to something bad. Through the wrong kinds of thoughts, the devil will try to infiltrate our thinking to lead us into problems.
No one would immediately accept or yield to a thought like, "I should throw my Bible away" or "I would be better off if I never went to church again". But, thoughts like, "I really don't have time to read my Bible again today, but that is OK, I can read my Bible tomorrow, or the day after that", or "Church is nice, but I have been so busy I need a day to rest, if I miss church just once or twice it won't matter". Those kinds of thoughts don't seem so bad, but they can lead to other worse thoughts, which will lead us into trouble. By checking all of the thoughts we have by what we see in God's Word, be can be sure that no wrong thoughts will infiltrate our mind and cause us problems.