It is a principle of grace savingly wrought, whereby the heart becomes holy, and is made after God’s own heart. A sanctified person bears not only God’s name—but his image. In opening the nature of sanctification, I shall lay down these seven positions:
(1.) Sanctification is a SUPERNATURAL thing; it is divinely infused. We are naturally polluted, and to cleanse, God takes to be his prerogative. “I am the Lord, who sanctifies you.” Lev 21:8. Weeds grow by themselves. Flowers must be planted and cultivated. Sanctification is a flower of the Spirit’s planting, therefore it is called, “The sanctification of the Spirit.” 1 Pet 1:2.
(2.) Sanctification is an INTERNAL thing; it lies chiefly in the heart. It is called “the adorning the hidden man of the heart.” 1 Pet 3:4. The dew wets the leaf—but the sap is hidden in the root. Just so, the religion of some consists only in externals—but sanctification is deeply rooted in the soul. “In the hidden part you shall make me to know wisdom.” Psalm 51:6.
(3.) Sanctification is an EXTENSIVE thing: it spreads into the whole man. “May the God of peace sanctify you wholly.” 1 Thess 5:23. As original corruption has depraved all the faculties—”the whole head is sick, the whole heart faint,” no part sound, as if the whole volume of blood were corrupted; just so, sanctification goes over the whole soul. After the fall, there was ignorance in the mind; but in sanctification, we are “light in the Lord.” Eph 5:8. After the fall, the will was depraved; there was not only impotence to good—but obstinacy. In sanctification, there is a blessed pliableness in the will, with the will of God. After the fall, the affections were misplaced on wrong objects; in sanctification, they are turned into a sweet order and harmony—the grief placed on sin, the love on God, the joy on heaven. Thus sanctification spreads itself as far as original corruption; it goes over the whole soul. “May God of peace sanctify you wholly.” He is not a sanctified person who is good only in some part—but who is all over sanctified; therefore, in Scripture, grace is called a “new man,” not a new eye or a new tongue—but a “new man.” Col 3:10. A good Christian, though he is sanctified but in part—yet in every part.
(4.) Sanctification is an intense and ARDENT thing. Its properties burn within the believer. “Fervent in spirit.” Rom 12:2. Sanctification is not a dead form—but it is inflamed into zeal. We call water hot, when it is so in the third or fourth degree. Just so, he is holy whose true religion is heated to some degree, and his heart boils over in love to God.
(5.) Sanctification is a BEAUTIFUL thing. It makes God and angels fall in love with us. “The beauties of holiness.” Psalm 110:3. As the sun is to the world, so is sanctification to the soul, beautifying and bespangling it in God’s eyes. That which makes God glorious must needs make us so. Holiness is the most sparkling jewel in the Godhead. “Glorious in holiness.” Ex 15:11. Sanctification is the first fruit of the Spirit; it is heaven begun in the soul. Sanctification and glory differ only in degree. Sanctification is glory in the seed; and glory is sanctification in the flower. Holiness is the quintessence of happiness.
(6.) Sanctification is an ABIDING thing. “His seed remains in him.” 1 Jn 3:9. He who is truly sanctified, cannot fall from that state. Indeed, mere seeming holiness may be lost—colors may wash off. Sanctification may suffer an eclipse. “You have left your first love.” Rev 2:4. True sanctification is a blossom of eternity. “The anointing which you have received, abides in you.” 1 Jn 2:27. He who is truly sanctified can no more fall away, than the angels which are fixed in their heavenly orbs.
(7.) Sanctification is a PROGRESSIVE thing. It is growing; it is compared to seed which grows: first the blade springs up, then the ear, then the ripe corn in the ear. Such as are already sanctified may be more sanctified. 2 Cor 7:1. Justification does not admit of degrees; a believer cannot be more elected or justified than he is—but he may be more sanctified than he is. Sanctification is still increasing, like the morning sun, which grows brighter to the full meridian. Knowledge is said to increase, and faith to increase. Col 1:10; 2 Cor 10:5. A Christian is continually adding an inch to his spiritual stature. It is not with us as it was with Christ, who received the Spirit without measure; for Christ could not be more holy than he was. We have the Spirit only in measure, and may be still augmenting our grace; as Apelles, when he had drawn a picture, would be still mending it with his pencil. The image of God is drawn but imperfectly in us, therefore we must be still mending it, and drawing it in more lively colors. Sanctification is progressive; if it does not grow—it is because it does not live. Thus you see the nature of sanctification.
Thomas Watson, A Body of Divinity