This is something that should be basic knowledge. According to the Bible, according to doctrinal Christian belief, man is evil at his heart and center and core. Any man who believes otherwise is probably not a Christian. The verse in question is Jeremiah 17:9. For my own purposes it’s in many different translations, to get a clear idea of the author’s intent.
The heart is deceitful above all things
and beyond cure.
Who can understand it?