“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).
If the “corruption of nature” has been truly mortified then how can the WCF assert that the hearts of true believers are deceitful (cf. Of Providence, V. 5)? Evidently, this “mortification” does not include a complete death of the old heart (i.e., its removal), nor a complete vivification that includes the replacement of the old deceitful heart of stone with a regenerate heart of flesh (compare Ezekiel 36:26 and Jeremiah 17:9).