Give me chastity and continence, but not quite yet.   ~Augustine  
You could say something supernatural happens the moment we place our trust in the finished work of Jesus on his cross of shame and suffering as evidenced by our confessing we are a sinner and by our denouncing all other allegiances.  And although often times it may not look like we are changing all that much to those who are much too busy inspecting our fruit (while their own spoiled bananas are rotten and rancid)—we can be sure God’s own will be a changed people.  Faith without works will always be dead.  Satan himself believes in Jesus and it makes him no follower.
The reality is that as Christians we do change.  But the change is many times not the way we might guess or assume—it sure takes longer than we’d like—and worse yet the biggest obstacle to our changing is the person we see in the mirror every morning when we brush our teeth.  I am in no way suggesting that we stay the same—because we do not—but the point I am making is an important one.  If we are honest, we have to confess that we still dabble with sin—we nibble and even splurge.  No matter—sin is always over-indulging.  If we are really honest, we’d be willing to admit that we wallow around in it at times and privately wonder how as a Jesus-follower we could do such a thing.  You can’t deal with what you don’t acknowledge.  Sin hasn’t packed up his bags and left us quite yet—we may have thought he left for good when we shooed him out the door last, but he’s been merely hiding around the corner patiently waiting for a more opportune time to clear his throat and whisper sweet nothings in our ear.  To believe otherwise is only to deceive ourselves and worse yet—to mislead our younger brothers and sisters in Christ. 
We just get in the way sometimes.  The not-so-natural but inevitable progression when we become Jesus-followers is that we begin to do away with our old ways of living and thinking.  It’s not that we must be someone we are not—no—we have been given a brand new permanent identity.  Challenging to say the least—and just like old habits—our old identity doesn’t die so quickly.  It is the miracle after all that transpires at the very instant of our conversion which makes the absolute difference and it is this: God comes to dwell within us via the Holy Spirit who is the Third Person in the Trinity (for those who may be new to this whole idea).   God himself living within us.  I didn’t say we become God.  Big difference. 
The mark of a true follower of Jesus isn’t an unchanged heart but a changed one—and a changed heart is only seen through a changed life.  I started this short blog thread titled In Process based on the premise that following Jesus is just that—a process—which according to Merriam-Webster is A natural phenomenon marked by gradual changes that lead toward a particular result. 
 3 His divine power has granted to us all things that pertain to life and godliness, through the knowledge of him who called us to his own glory and excellence, 4 by which he has granted to us his precious and very great promises, so that through them you may become partakers of the divine nature, having escaped from the corruption that is in the world because of sinful desire. 5 For this very reason, make every effort to supplement your faith with virtue, and virtue with knowledge, 6 and knowledge with self-control, and self-control with steadfastness, and steadfastness with godliness, 7 and godliness with brotherly affection, and brotherly affection with love. 8 For if these qualities are yours and are increasing, they keep you from being ineffective or unfruitful in the knowledge of our Lord Jesus Christ.   (2 Peter 1:3-8, ESV)
Notice Peter—the very man who denied even knowing Jesus on the eve of his betrayal—writes if these qualities are yours and are increasing…  Peter’s growth in grace didn’t end just because he stumbled again—he went on to be changed over and over again.  It can seem as though it is two steps forward and one step backwards at times—if anyone knew a thing or two about gradual change, it was Peter himself.  I’ve never liked the old saying Christians aren’t perfect just forgiven because it falls so grossly short in painting a picture of just who us Christians are.  Cheesy little maxims never do summarize what God has really said—they might sell some bumper stickers, but that’s about it. 
Christians are surely forgiven—but that’s not all they are if they are true-blue followers of the Son of God—they are changed in the process.