I like how Charles Hodge starts his discussion on sanctification in his Systematic Theology:
Sanctification in the Westminster Catechism is said to be “the work of God’s free grace, whereby we are renewed in the whole man after the image of God, and are enabled more and more to die unto sin and live unto righteousness.”
Agreeably to this definition, justification differs from sanctification, (1) In that the former is a transient [one time] act, the latter a progressive work. (2) Justification is a forensic act, God acting as judge, declaring justice satisfied so far as the believing sinner is concerned, whereas sanctification is an effect due to the divine efficiency. (3) Justification changes, or declares to be changed, the relation of the sinner to the justice of God; sanctification involves a change of character. (4) The former, therefore, is objective, the latter subjective. (5) The former is founded on what Christ has done for us; the latter is the effect of what He does in us. (6) Justification is complete and the same in all, while sanctification is progressive, and is more complete in some than in others.
Sanctification is declared to be a work of God’s free grace. Two things are included in this. First, that the power or influence by which it is carried on is supernatural. Secondly, that granting this influence to any sinner, to one sinner rather than another, and to one more than to another, is a matter of favor. No one has personally, or in himself, on the ground of anything he has done, the right to claim this divine influence as a just recompense, or as a matter of justice.