The English definition of "heart" is "center of life". We are looking at this word in its spiritual sense. Your spiritual man has a heart that can never be separated. It's where the revelation knowledge of God is stored in your spirit man, the true you.
I'm going to give you 2 Corinthians 5:17 from several versions of the Scriptures which each say it in a different way. When a Scripture says that we're a "new creation", it's an incorrect interpretation ... we are new creatures, in our spirits. We have not been made a whole new creation.
God made our spirits ... which were dead to Him (separated from) Him ... alive unto Him.
NKJV ... Therefore if anyone is in Christ, he is (made new); old things have passed away; behold, all things have become new.
NLT ... This means that anyone who belongs to Christ, has become a new person. The old life is gone; a new life has begun!
THE MESSAGE BIBLE ... Anyone who's united with the Messiah gets a fresh start, is made new. The old life is gone, a new life burgeons (begins)!
AMPLIFIED VERSION ... If any person is (ingrafted) in Christ, he is a new creature altogether; the old previous moral and spiritual condition has passed away. Behold, the new and fresh has come!
Reverend Robert W. & Mary C. Butler