“…taking away their heart of stone, and giving unto them an heart of flesh” (X.1).
The WCF says that those who are effectually called, have their old hearts of stone taken away, and a new heart of flesh in given in its place:
“A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh” (Ezekiel 36:26).
But since the WCF also says that a Christian’s heart is deceitful (cf. V.5) then what exactly was taken away? What exactly was given in its place? If a deceitful heart still remains, then how do they know that the deceitful, stony heart has been taken out or not? How do they know that their deceitful heart of stone—that according to them has NOT been taken out and replaced with a heart of flesh—is not deceiving them into thinking that their heart of stone has been replaced with a heart of flesh?
This is not to say that Christians do not have remaining (indwelling) sin. Indeed they do (see Romans 7:17).
The conclusion drawn from WCF V.5 and X.1 regarding the heart of regenerate persons is that the principle of indwelling sin of Romans 7:17 is what is deceitful and desperately wicked. Thus, while the authors of the WCF superficially affirm the truth of Ezekiel 36:26, they in reality reject an orthodox interpretation of it.