How are we to understand the nature of sanctification in the life of a believer in Jesus Christ? What is the relationship between the work that God is presently carrying on to completion in our lives (Philippians 1:6), and the full and final completion of that work in us at the day of Christ?
In his book, A Body of Divinity, Thomas Watson writes,
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. (p.242)
Glory is the full flower of which sanctification is the seed. As Watson says, they differ only in degree, not in kind.
So our sanctification (the ongoing work of God’s grace in our lives whereby we are “enabled more and more to die unto sin, and live unto righteousness” – Westminster Shorter Catechism Q.35) is the beginnings of the glory that we will finally and fully enjoy in heaven one day.
When seen in that particular light, the effort expended on our part in dying to sin and living unto righteousness in this life can be seen not as a burden to bear, but as a blessing to enjoy. It is not only evidence that our citizenship truly is in heaven (Philippians 3:20), but is also then (in a sense) a foretaste of the perfect holiness and happiness that will be ours to enjoy forever in heaven with the Lord Jesus Christ.