"What is it that Paul says he is afraid of here? He isn’t writing because he fears some of the Corinthians are on the verge of committing perjury or embezzling thousands of dollars or killing someone. He is afraid their “thoughts will be led astray from a sincere and pure devotion to Christ.” This is where all of the Big Ten horrible, obvious sins begin–with our thoughts going astray. When it comes to thoughts, does Satan really have to work very hard to start those on the wrong path?  

Sincerity and purity are closely related and can sometimes be used as synonyms. Both have elements of simplicity. Something pure is simple because it is not tainted with anything. It is unadulterated. If I pour myself a glass of water that has passed through my water filter, then add a tiny grain of arsenic, drinking the water may not kill me outright, but the water can no longer be considered pure."