Changed from the Inside Out by Max Lucado

When you believe in Christ, Christ works a miracle in you. You are permanently purified and empowered by God himself. The message of Jesus to the religious person is simple: It's not what you do. It's what I do. I have moved in. And in time you can say with Paul, "I myself no longer live, but Christ lives in me" (Gal. 2:20).
If I'm born again, why do I fall so often?
Why did you fall so often after your first birth? Did you exit the womb wearing cross-trainers? Did you do the two-step on the day of your delivery? Of course not. And when you started to walk, you fell more than you stood. Should we expect anything different from our spiritual walk?
But I fall so often, I question my salvation. Again, we return to your first birth. Didn't you stumble as you were learning to walk? And when you stumbled, did you question the validity of your physical birth? Did you, as a one-year-old fresh flopped on the floor, shake your head and think, I have fallen again. I must not be human?
Of course not. The stumbles of a toddler do not invalidate the act of birth. And the stumbles of a Christian do not annul his spiritual birth.
Do you understand what God has done? He has deposited a Christ seed in you. As it grows, you will change. It's not that sin has no more presence in your life, but rather that sin has no more power over your life. Temptation will pester you, but temptation will not master you. What hope this brings!
Hear this. It's not up to you! Within you abides a budding power. Trust him!
Think of it this way. Suppose you, for most of your life, have had a heart condition. Your frail pumper restricts your activities. Each morning at work when the healthy employees take the stairs, you wait for the elevator.
But then comes the transplant. A healthy heart is placed within you. After recovery, you return to work and encounter the flight of stairs--the same flight of stairs you earlier avoided. By habit, you start for the elevator. But then you remember. You aren't the same person. You have a new heart. Within you dwells a new power.
Do you live like the old person or the new? Do you count yourself as having a new heart or old? You have a choice to make.
You might say, "I can't climb stairs; I'm too weak." Does your choice negate the presence of a new heart? Dismiss the work of the surgeon? No. Choosing the elevator would suggest only one fact--you haven't learned to trust your new power.
It takes time. But at some point you've got to try those stairs. You've got to test the new ticker. You've got to experiment with the new you. For if you don't, you will run out of steam.
Religious rule keeping can sap your strength. It's endless. There is always another class to attend, Sabbath to obey, Ramadan to observe. No prison is as endless as the prison of perfection. Her inmates find work but never find peace. How could they? They never know when they are finished.
Christ, however, gifts you with a finished work. He fulfilled the law for you. Bid farewell to the burden of religion. Gone is the fear that having done everything, you might not have done enough. You climb the stairs, not by your strength, but his. God pledges to help those who stop trying to help themselves.

"He who began a good work in you will carry it on to completion until the day of Christ Jesus" (Phil. 1:6.)

God will change you from the inside out.