The believer may not know how much damage associating with the world's system is having on him until he sees the content of his heart. Associating with evil corrupts good and godly manner.
Scripture admonishes us to flee from the system governing the world. "Love not the world." Loving the world is enmity with God.
The believer is in the world but not of the world. It is just fine to see the boat floating on water but an anomaly to see the water in the boat.
Thank you @deathlyhorror
Great is your insight.