There is... a distinction when the matter is considered in reference to Christ. Christ is our justification, considered as hanging upon the cross, and enduring the penalty of the law for us. In other words, Christ is our justification by standing in our stead, and by receiving in his own person the stripes and chastisement, by which those who have sinned are healed. Christ is our sanctification, (that is, the cause or ground of our sanctification,) considered as operating and living in us by the present and efficacious influences of the Holy Spirit, which he has purchased by his blood. In both cases, Christ is the ground or efficacious cause of the result; and in both cases, also, there is something done inwardly as well as outwardly. But it is nevertheless true, that in justification the work, which is done. is done in a peculiar sense exteriorly, or FOR men; while the work of sanctification is done, in an equally peculiar and emphatic sense, interiorly, or WITHIN them.
Another mark of distinction is, that sanctification is regarded, and very properly regarded, as an evidence of justification. They have not only the relation of antecedence and sequence in the order of time, but the additional and incidental relation of fact and evidence. In other words, the sanctification of a person holds the relation of evidence or proof to the alleged fact of his being justified. That there is good foundation for this view, additional to its innate reasonableness, seems to be evident from the repeated instructions of the Savior, that men are known by their fruits. And certainly, we may most reasonably expect, that he, who has been justified, will aim to bear the fruits of a holy life. Having been instructed by the Holy Spirit in the nature and tendencies of sin, and having found in the Gospel that redemption which he could find no where else, how is it possible that he should again sin against God? Hence it is that he seeks for sanctifying grace, and endeavors to purify himself from every form of iniquity. And it is a matter of common and agreed opinion, that he, who is careless in respect to sanctification, has no satisfactory evidence that he is truly justified.
Justification, when it has taken effect, is a thing which is done or completed; at least in such a sense as to exclude the idea of its being a progressive work. As we have already stated, it looks only to the past; but in its relation to the past it is complete. The result of its application, in any given case, is, that the multiplied sins, which have been committed in former times, are blotted out. If we sin at the present moment, and justification is immediately applied, it is still true, that the sin, in the order of nature and in reference to the time of justification, however closely the justification may follow the sinful act, is a past sin. Justification must necessarily be subsequent, and consequently the sin, relatively to the time of justification, must necessarily be past, even in those cases in which, in common parlance, we speak of the sin as a present sin. The work of justification, therefore, when it has once taken place, is a thing complete in itself, and is not in its own nature susceptible of progress, although it may be necessary to have it repeated in every succeeding moment.
Sanctification, on the other hand, is a thing which is indwelling, permanent, and always progressive. It is not only progressive, until all the evils of the heart are subdued, but even when it is in some degree complete, so much so as to occupy the whole extent of our being, and to substitute in the heart everywhere good for evil, it is still progressive in DEGREE. So that in those cases where we speak of sanctification as entire, it is still true that its entireness is not such as to exclude progress. There will never be a period, either in time or eternity, when there may not be an increase of holy love.
— The Interior or Hidden Life (1844) Part 2, Chapter 1.