“This corruption of nature, during this life, doth remain in those that are regenerated; and although it be through Christ pardoned and mortified, yet both itself and all the motions thereof are truly and properly sin” (VI.5).
The WCF says that the “corruption of nature” is mortified through the work of Christ. But since the men of Westminster believed that their own hearts are deceitful, then it is simply their unregenerate and deceitful hearts that are deceiving them into thinking that their corrupt nature has been truly mortified.
It is asserted that those who are effectually called and regenerated, have “a new heart and a new spirit created in them” (Of Sanctification, XIII.1). But the WCF men do not believe the biblical teaching regarding this new heart and new spirit. For the Bible teaches that the new regenerate heart and spirit replaces the old deceitful and desperately wicked unregenerate heart and spirit.
If the “corruption of nature” has been mortified then how can it be said in V.5 (Of Providence) that the hearts of true believers are deceitful? Evidently, this “mortification” does not include a complete death of the old heart (with its removal), nor a complete vivification with the replacement of the old deceitful heart of stone with a regenerate heart of flesh (Ezekiel 36:26-27).
It is true according to Romans 7:14-25 that the Christian still retains a sin principle (not a sin nature), but this “sin principle” is not deceitful above all things, nor desperately wicked contrary to the deceitful and desperately wicked teaching of the WCF.