It sounds like I'm giving a formula for salvation and Christian living - the sinner repents, he is then justified by God, and goes on to live a life of practical sanctification. But that's not what I want to address here. I am not going to talk about our repentance, justification or sanctification, but God's!
You see, people have handy definitions for these three words (repent, justification, and sanctification), but in Scripture these three words are applied to God and cannot mean what people often say they mean.
When we think of repentance, we often think of it as meaning "turning from sin", and, if properly understood, that is ok as a description of what it means for us to repent, but it isn't a definition of the word. The word means a change of mind or a change of attitude. What it means in a Gospel context is that the sinner has to change his attitude to his sin and see it as something hateful and hell-deserving in God's sight, and he has to change his attitude to himself and acknowledge that he is helpless to save himself - he needs a Saviour. But the word is sometimes applied to God, and what it means is that when a person changes his attitude toward God, God will change His attitude, as it were, toward him, (see here for more on God repenting.)
Justification is often thought of as having one's guilt removed, and again, when applied to the sinner that is what happens (among other things) when they are justified, but that's not what the word actually means. The word "justify" means to declare to be righteous, it does not mean to make righteous. Thus God gives instructions to Israel's judges to justify the righteous (Deuteronomy 25 v 1). Keeping this in mind helps us when we compare Paul and James on the subject. Paul is saying that we are declared righteous (justified) legally by faith in Christ, but James is saying we are declared righteous practically by works which evidence our faith.
So in Romans 3 v 4 God is said to be justified, and the point is that God righteousness is declared both in the judgment of the unrepentant and in the salvation of the repentant. This is one of the many unique things of the Gospel - God is just and the justifier of those who believe on the Lord Jesus (Romans 3 v 26).
When people think of the subject of sanctification often the idea many of them have is that of becoming more holy, and again, in certain contexts that is what is in view, but it's not the meaning of the word. Sometimes inanimate objects are sanctified (e.g. here), some unbelievers are sanctified (e.g. here), carnal Christians are sanctified (e.g. here), and God and the Lord Jesus Christ are sanctified (e.g. here). Thus the word cannot have the definition of "make more holy" - the word simply means "set apart".
For believers this has at least a two-fold application. Upon trusting Christ to save us we are sanctified positionally. That means we are separated from our sins and set apart as belonging to God (e.g. 1 Corinthians 1 v 2; Hebrews 10 v 10). In this sense, the newest, most immature Christian is just as sanctified as the godliest, most mature Christian - there are no levels or degrees of positional sanctification. But there is a second sense of the word, and that is that we are to be sanctified practically, that is, in our practice we are to live lives separate from sin and devoted to God (e.g. 1 Thessalonians 4 v 3-4; 2 Timothy 2 v 21).
In John 10 v 36 the Lord says He was set apart by the Father and sent into the world - the reason He was set apart, and not a mere man or mighty angel, was because only one who was divine could live the life that He lived - fully representing God's character, and only one who was divine could die the death that He died - fully satisfying God's justice.
The Lord was not only set apart at His coming into the world, but He set Himself apart at His going out of the world (John 17 v 19), and is in heaven helping, enabling and encouraging His people.
