Anyone who believes in what Jesus did for them (forgive all sins, forever reconciled to God) becomes a new creation in their true self, in their spirit.
Prophetically announced by God 600 years before Jesus came to earth:
“I will give you a new heart and put a new spirit in you; I will remove from you your heart of stone and give you a heart of flesh.” (God in Ezekiel 36:26; NIV)
This is not an assumption, but biblically based certainty.
And above all, if this weren’t the case, then we wouldn’t be fit for heaven, for living together with God. Jesus makes it clear:
“Very truly I tell you, no one can see the kingdom of God unless they are born again.” (Jesus in John 3:3; NIV)