Justification and Sanctification
Let me show you the essential difference between justification and sanctification. Look at it like this: Justification is an act of God the Father; sanctification is essentially the work of God the Holy Spirit. There is this division of work in the blessed Persons of the Trinity. It is the Father who declares righteous and just. It is the Holy Spirit who sanctifies.
Second, justification takes place outside us, as in a tribunal; sanctification takes place within us, in our inner life. I stand in the court when I am justified, and the judge pronounces that I am free; it is a statement about me, outside me. But sanctification is something that is worked and takes place within.
Third, justification removes the guilt of sin; sanctification removes the pollution of sin and renews us in the image of God.
And therefore, last, by definition justification is a once-and-for-all act. It is never to be repeated because it cannot be repeated and never needs to be repeated. It is not a process but a declaration that we are pronounced just once and forever, by God. Sanctification, on the other hand, is a continuous process. We continue to grow in grace and in the knowledge of the Lord until we are perfect beyond the veil.
So there is nothing quite so erroneous and confusing and unscriptural as to mistake the essential difference between justification and sanctification. That is the whole trouble with Roman Catholic teaching and all Catholic piety. If you confuse sanctification with justification, you will be doubtful as to whether you are justified or not. If you bring in your state and condition and sin that you may commit, then you are querying your justification. But if you realize that justification is forensic, external, and declaratory, you know that you are justified whatever may be true about you.
– Martyn Lloyd-Jones Great Doctrines of the Bible: God the Father, God the Son – God the Holy …