Thomas Boston (d. 1732), a Scottish Presbyterian pastor, wrote numerous books and treatises as well as a commentary on the Westminster Shorter Catechism (called “An Illustration of the Doctrines of the Christian Religion”). Below is a helpful excerpt from his section on sanctification (which I’ve edited to keep it brief).
First: Sanctification of a soul is twofold. 1) Initial sanctification, and 2) Progressive sanctification.
Second: The Author of sanctification. 1) Negatively, the author of it is not the sinner himself nor any other creature. Positively, it is the work of God; it is the work of the Triune God to sanctify a soul, but in a special manner it belongs to the Holy Spirit (2 Thes. 2:13). In progressive sanctification, the sinner does act towards his own sanctification (2 Cor. 7:1) yet he acts not but as he is acted by the Holy Spirit (Phil. 2:13).
Third: The moving cause of sanctification. The only cause of it is free grace, not any personal worth in the creature (Titus 3:5).
Fourth: Of what sanctification consists. It consists in the Holy Spirit renewing the sinner after the image of God (Eph. 4:23-24). It consists of destroying the old man within, the body of sin (Rom. 6:6, Col. 2:11). It consists of endowing the sinner with all the graces of the Spirit (John 1:16). This is the new man which is put on in sanctification.
Fifth: The two parts of sanctification. 1) Mortification, whereby the sinner is enabled more and more to die unto sin (Rom. 6:4, 6). 2) Vivification, whereby the sinner is enabled more and more to live unto righteousness (Rom. 6:4).
Sixth: The subjects of sanctification. 1) It is the elect who are sanctified, even all of them, and they only (Eph. 1:4, 2 Thes. 2:13). Sanctification is a certain evidence of election. 2) The whole person is sanctified (2 Cor. 5:17, 1 Thes. 5:23). The grace of sanctification is a holy leaven that goes through the whole lump, and makes every part of the man holy, soul and body. Although the whole man is sanctified, yet no part of the man is perfectly sanctified in this life.
Seventh: The effect of sanctification. Holiness is the effect; the fruit of this work of the Spirit is an habitual aversion of the soul to evil and an inclination to good.
Eighth: How man is sanctified. It is a work carried on by degrees. The Spirit implants grace in the soul, giving it a new power towards God and his law. The Spirit preserves the implanted grace (1 Pet. 1:5). The Spirit excites and quickens this implanted grace in the soul; he strengthens and excites it and quickens it to resist the flesh and do battle with evil. At death he perfects the work (Heb. 12:23).
Finally: The means of sanctification. The outward means that the Spirit makes use of are ordinances (public and private), especially the word and sacraments (Eph. 5:26). And they that would be sanctified must use these means of sanctification, whereby the Spirit begins and carries on the work. He also uses providences; both favorable ones and afflictions are used by the Spirit to sanctify.
This summary of Boston’s exposition of sanctification is found in “An Illustration of the Doctrines of the Christian Religion, Part 1,” which, in turn, can be found in the first volume of Boston’s complete works.