The Bible commonly uses the term sanctify (The same Greek word is behind the words sanctification, saint, holy) to mean set apart from sin to God, to be holy. A Christian's sanctification has three aspects: past (positional justification), present (progressive sanctification), and future (perfect glorification). We know that justification and glorification are by God's grace through faith, not our effort or works. Can we say the same about our present experience of sanctification?
Sanctification (we will use the word to mean present progressive sanctification) is by grace because the God who justified us also provides everything we need on the way to our final glorification (Rom. 8:29-32). The three persons of the Godhead all play an active role in our sanctification: The Father (John 17:17; 1 Thes. 5:23), the Son (Eph. 5:26; 1 John 1:7); and the Spirit (Rom. 15:16; 2 Cor. 3:18). God also uses various means for our sanctification such as His Word, His Spirit, the church, trials, and various other experiences. The power of His Holy Spirit that gives us new birth at justification is the same power that sanctifies us through the life of the risen Christ.
Since God supplies the power, sanctification is by grace, but it is not automatic. If it were, it would seem that all Christians would grow at the same rate and none could be held accountable for stagnation or lack of growth. But we know that all Christians do not grow at the same rate or progress to the same degree of holiness. We also know that the Judgment Seat of Christ holds Christians accountable for how they use their lives (Rom. 14:10-12; 2 Cor. 5:9-10), which indicates different degrees of progress in sanctification.
That is why many Bible passages put the onus on the Christian to grow spiritually (eg. Rom. 12:1-2; 2 Peter 3:18). However, the Bible clearly indicates that the Christian must cooperate with God.
1 Corinthians 15:10. But by the grace of God I am what I am, and His grace toward me was not in vain; but I labored more abundantly than they all, yet not I, but the grace of God which was with me.
Philippians 2:12-13. Therefore, my beloved, as you have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling; for it is God who works in you both to will and to do for His good pleasure.
Colossians 1:29. To this end I also labor, striving according to His working which works in me mightily.
2 Peter 1:3-4 with 5-6. ...His divine power has given to us all things that pertain to life and godliness, through the knowledge of Him who called us by glory and virtue, by which have been given to us exceedingly great and precious promises ...But also for this very reason, giving all diligence, add to your faith virtue, to virtue knowledge, to knowledge self-control...
An illustration may help understand the cooperation required. For a baby girl to grow, parents must feed her. But it is also true that for a baby girl to grow, she must eat. Both of these statements are true, because a baby's growth is a cooperative effort.
God's grace in sanctification is available in the form of every resource we need to grow in Christ. But like eternal salvation, these gracious resources must be accessed through faith.
Romans 5:1-2. Therefore, having been justified by faith, we have peace with God through our Lord Jesus Christ, through whom also we have access by faith into this grace in which we stand ...
Galatians 2:20. I have been crucified with Christ; it is no longer I who live, but Christ lives in me; and the life which I now live in the flesh I live by faith in the Son of God, who loved me and gave Himself for me.
Faith claims and appropriates the power and promises of God that bring spiritual growth.
Every Christian is sanctified by grace from the moment of justification, through present sanctification, to the final state of glorification. But the progress of our present sanctification may vary depending on our faith that accesses the grace God has made available to us in Christ. Sanctification is God's desire for us (1 Thes. 4:3; Heb. 12:14; 1 Peter 1:14-15), but it is not an automatic act of God, nor is it merely from human effort. Christians are sanctified by God's grace accessed through faith.