Whenever we do not view all scripture through the filter lens of the finished work of Christ "after" the cross and resurrection - we are going to have confusion.
Yes, Jesus did say that out of the heart flows all kinds of evil things which is why we need a new heart. Jesus said in scripture "in context" to "clean the inside of the cup and then the outside will become clean". Jesus did clean the heart of man but it comes by faith - not by human effort to make oneself better and supposedly righteous and pure. That's called a religion.
Here in the first few lines I see a non-biblical assumption that is clearly wrong.
The heart of a person is the place where they dwell. It is their essence, their
actual thoughts, attitudes, reactions without a filter.
Scripture calls that we are a new creation, born again, so a real new beginning born
of God.
Ezek talks about a new heart in terms of attitude approach.
Now the new heart is a heart of flesh and not of stone. So this heart is not defended,
love, empathy, openness can work within it. It is not necessary a transplant but a
change of being.
Now our experience is our desire can still flow carnally, and we can still dwell on the
wrong things inside ourselves, in our hearts.
So if you go with a new heart theology, the carnal side is the flesh or the old heart.
But then ones language can just get schizophrenic, and everything good is of God,
and everything evil is the flesh or the old heart.
Paul talks about the blood of christ purifying us, our consciences being cleaned etc.
It is why the language of the new heart sounds gnostic and shows all the similar theological
failings of instant perfection, realisation of the who we are not becoming what we need to
be in Christ, the ultimate fulfillment.
And all the rejection of Christs words and victory, purity and righteousness, the condemnation
of religion as a mission etc all stems from this mistake.
Therefore, if anyone is in Christ, the new creation has come: The old has gone, the new is here!
2 cor 5:17
A man reaps what he sows. Whoever sows to please their flesh, from the flesh will reap destruction; whoever sows to please the Spirit, from the Spirit will reap eternal life.
Gal 6:7-8
Sowing to the flesh is not immature Christians but falling into hell.
So by diluting what the flesh means, creating a split of heart, perfect new, flawed old, a different
faith is created. It is why the common words are redefined, and spiritual realities condemned as
failed religion, while everything is new mystical grace realisation.
If I saw this in scripture, I would follow it, but it is not there and only works by making Jesus's
words irrelevant and empty.