The Devil has become fully convinced of the righteousness and validity of his own cause. The father of lies has become the truest believer in his own lies. In his blindness, satan has become of the utmost sincerity in his deception. Therefore, he can infiltrate the body of Christ both through insincere and sincere men alike – who at times may speak even the purest of words, but in the most manipulative spirit, whether wittingly or unwittingly..
Without the Holy Spirit being always allowed to cut us asunder, revealing the unthought thoughts and unintended intentions of the heart; we are hopelessly outmatched by and easily made a mouthpiece of the subtlety of the serpent.
Those who know the Truth as the Person of Jesus have little regard for sincerity and much regard for being true in Christ. For the two are quite distinct, as much as they ought to be intertwined.