It is the will of God that we should be holy, as saith the text, ‘This is the will of God, your sanctification’ (1 Thess 4:3).  As God’s word must be the rule, so his will, the reason of our actions. This is the will of God, our sanctification. Perhaps it is not the will of God we should be rich, but it is his will that we should be holy. God’s will is our warrant.

1. Jesus Christ has died for our sanctification. Christ shed his blood to wash off our impurity. The cross was both an altar and a laver. ‘Who gave himself for us to redeem us from all iniquity.’ Tit 2:14. If we could be saved without holiness, Christ needed not have died. Christ died, not only to save us from wrath, but from sin.
2. Sanctification makes us resemble God. It was Adam’s sin that he aspired to be like God in omniscience, but we must endeavour to be like him in sanctity. It is a clear glass in which we can see a face; it is a holy heart in which something of God can be seen. Nothing of God can be seen in an unsanctified man, but you may see Satan’s picture in him. Envy is the devil’s eye, hypocrisy his cloven foot; but nothing of God’s image can be seen in him.
3. Sanctification is that which God bears a great love to. Not any outward ornaments, high blood, or worldly grandeur, draws God’s love, but a heart embellished with holiness does. Christ never admired anything but the beauty of holiness: he slighted the glorious buildings of the temple, but admired the woman’s faith, and said, ‘O woman, great is thy faith.’…As a king delights to see his image upon a piece of coin, so where God sees his likeness he gives his love. The Lord has two heavens to dwell in, and the holy heart is one of them.
4. Sanctification is the only thing that makes us differ from the wicked. God’s people have his seal upon them. ‘The foundation of God standeth sure, having this seal, The Lord knoweth them that are his. And, Let every one that nameth the name of Christ depart from iniquity.’ 2 Tim 2:19. The godly are sealed with a double seal, a seal of election, ‘The Lord knoweth who are his,’ and a seal of sanctification, ‘Let every one that nameth the name of Christ depart from iniquity.’ This is the name by which God’s people are known, ‘The people of thy holiness.’ Isa 63:18. As chastity distinguishes a virtuous woman from a harlot, so sanctification distinguishes God’s people from others. ‘Ye have received an unction from the Holy one.’ 1 Jn 2:20.
5. It is as great a shame to have the name of a Christian, yet want sanctity, as to have the name of a steward and want fidelity; or the name of a virgin, and want chastity. It exposes religion to reproach, to be baptized into the name of Christ while unholy, and to have eyes full of tears on a sabbath, and on a week-day eyes full of adultery:2 Pet 2:14; to be so devout at the Lord’s table, as if men were stepping into heaven, and so profane the week after, as if they came out of hell; to have the name of Christians while unholy is a scandal to religion, and makes the ways of God evil spoken of.
6. Sanctification fits for heaven: ‘Who has called us to glory and virtue.’ 2 Pet 1:3. Glory is the throne, and sanctification is the step by which we ascend to it. As you first cleanse the vessel, and then pour in the wine; so God first cleanses us by sanctification, and then pours in the wine of glory. Solomon was first anointed with oil, and then was a king. 1 Kings 1:39. First God anoints us with the holy oil of his Spirit, and then sets the crown of happiness upon our head. Pureness of heart and seeing God are linked together. Mt 5:8.

(Thomas Watson, A Body of Divinity)