As David Platt explains it, "Justification is the gracious act of God by which He declares a sinner righteous only through faith in Jesus." However, sanctification is:"Sanctification is the process by which God transforms our lives into the image of Christ." The difference:"Justification is an instantaneous event; sanctification is a gradual process.
Justification is a one time event, not a process. Sanctification on the other hand is a process of pursuing holiness and becoming more like Christ. Of course we will sin because we are human. Yet, I believe it is only natural after being born again that we desire to be more like Christ.
I believe it is possible for a Christian to fall away, yet not possible to lose their salvation. When saved, always saved. The way I see it, if one is a believer and he or she falls away then God will bring him back to Him. If one never ever desires to pursue holiness and be like Christ, quite bluntly I wonder if they were ever converted in the first place.
"Is sanctification permanent?" Yes, from the moment Christ saves someone, sanctification is a process. It will not be made complete in this world. (1 John 1:8-10) (Romans 7:18-19) The great paradox is that although we have a sinful nature, we should strive to become more and more like Christ. Sanctification is running towards holiness and saying, "Because Christ died for me, I want to be more like Him! I want to behold His glory!" It's a way of life that demonstrates though none go with me, I still will follow!! It's not a drudgery of avoiding sins because it makes one more righteous. No, it's a transformation that God does, just like in justification. (1 Thess. 4:3-7) Our righteousness is as filthy rags to God. It is only by His grace alone, we can behold His glory and be conformed to the image of Him.
Here's a quote from John Murray on sanctification: Indeed, the more sanctified the person is, the more conformed he is to the image of his Savior, the more he must recoil against every lack of conformity to the holiness of God. The deeper his apprehension of the majesty of God, the greater the intensity of his love to God, the more persistent his yearning for the attainment of the prize of the high calling of God in Christ Jesus, the more conscious will he be of the gravity of the sin that remains and the more poignant will be his detestation of it......Was this not the effect in all the people of God as they came into closer proximity to the revelation of God's holiness?