Via JC Ryle's Holiness:Its Nature, Hindrances, Difficulties, and Roots, the difference between justification and holiness: "Justification is the reckoning and counting a man to be righteous for the sake of another, even Jesus Christ the Lord. Sanctification is the actual making a man inwardly righteous, though it may be in a very feeble degree."
Justification happens once and his how God sees people who truly follow Christ. Sanctification is ongoing, a process of becoming that which God sees. It reminds me of a different way to use the potter illustration. God as potter sees a lump of clay (His followers) as the design He originally intended. That's justification. God as potter works that lump of clay (His followers) to realize that design. That's sanctification.
