We have a new heart... and an old one (sort of).

Becoming a Christian, a born-again, Spirit indwelt, redeemed son of God immediately changes us. We are a new creation, freed from sin, with a new heart. Our desires, thinking, and spirit have all been made alive.  

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. -Ezekiel 36:26, Ezekiel 11:19

Don't these verses mean that we our old hearts are gone?  Well yes and no.  Salvation brings a new birth, new heart, new mind, new desires but we are still confined in these mortal bodies that are still subject to the desires of the flesh.  As Paul states in Romans 7:

For we know that the Law is spiritual, but I am of flesh, sold into bondage to sin. For what I am doing, I do not understand; for I am not practicing what I would like to do, but I am doing the very thing I hate. But if I do the very thing I do not want to do, I agree with the Law, confessing that the Law is good. So now, no longer am I the one doing it, but sin which dwells in me. For I know that nothing good dwells in me, that is, in my flesh; for the willing is present in me, but the doing of the good is not. For the good that I want, I do not do, but I practice the very evil that I do not want. But if I am doing the very thing I do not want, I am no longer the one doing it, but sin which dwells in me.

      I find then the principle that evil is present in me, the one who wants to do good. For I joyfully concur with the law of God in the inner man, but I see a different law in the members of my body, waging war against the law of my mind and making me a prisoner of the law of sin which is in my members. Wretched man that I am! Who will set me free from the body of this death? Thanks be to God through Jesus Christ our Lord! So then, on the one hand I myself with my mind am serving the law of God, but on the other, with my flesh the law of sin. -Romans 7:14-25

So while our hearts are new we still feel the desires of the flesh. Those desires no longer define us, but they still dwell in us. Before salvation our default was to sin and we desired to sin, but now our default is to hate sin and we desire to do good.  So why do we keep sinning?  

"Keep watching and praying that you may not come into temptation; the spirit is willing, but the flesh is weak." - Mark 14:28

We see that God's plan is to build us up in His power combined with our own will. He has given us His Spirit, living within us, and with the Spirit comes His power. If we have the Spirit and power of God within us that should be enough right?  Yes it is. But God has also seen fit to require something of us: our will. Just as salvation requires us to make a decision of the will to believe in Jesus, likewise our sanctification requires our will to daily deny the flesh.

And he said to all, “If anyone would come after me, let him deny himself and take up his cross daily and follow me. - Luke 9:23 

Our new hearts are the spiritual completion of creation.  The flesh needs daily food to sustain life, likewise our spirits need the daily food of the Word.  The flesh needs care and a community to thrive, likewise the new creation within us needs spiritual communion with God to thrive.