FIRST, be conversant in the word of God. “Sanctify them through thy truth,” John 17:17. The word is both a glass to show us the spots of our soul, and a laver to wash them away. The word has a transforming virtue in it; it irradiates the mind, and consecrates the heart.
Second, get faith in Christs blood. “Having purified their hearts by faith,” Acts 15:9. She in the gospel who touched the hem of Christs garment was healed. A touch of faith purifies. Nothing can have a greater force upon the heart, to sanctify it, than faith. If I believe Christ and His merits are mine, how can I sin against Him? Justifying faith does that in a spiritual sense which miraculous faith does, it removes mountains, the mountains of pride, lust and envy. Faith and the love of sin are inconsistent.
Third, breathe after the Spirit. It is called “the sanctification of the Spirit,” 2 Thessalonians 2:13. The Spirit sanctifies the heart, as lightning purifies the air, and as fire refines metals. The Spirit stamps the impression of His own sanctity upon the heart, as the seal prints its likeness upon the wax. The Spirit of God in a man perfumes him with holiness, and makes his heart a map of heaven.
Fourth, associate with sanctified persons. They may, by their counsel, prayers, and holy example, be a means to make you holy. As the communion of saints is in our creed, so it should be in our company. “He that walketh with the wise shall be wise,” Proverbs 13:20. Association begets assimilation.
Fifth, pray for sanctification. Job propounds a question. “Who can bring a clean thing out of an unclean?” Job 14:4. God can do it. Out of an unholy heart He can produce what is of grace. Oh! make Davids prayer your own, “Create in me a clean heart, O God,” Psalm 51:10. Lay thy heart before the Lord, and say, “Lord, my unsanctified heart pollutes all it touches. I am not fit to live with such a heart, for I cannot see thee. Oh create in me a new heart! Lord, consecrate my heart, and make it thy temple, and thy praises shall be sung there for ever.”
An edited extract from A Body of Divinity by Thomas Watson