The life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me. (Galatians 2:20)
Faith is a perfect fit with God’s future grace. It corresponds to the freedom and all-sufficiency of grace. And it calls attention to the glorious trustworthiness of God.
One of the important implications of this conclusion is that the faith that justifies and the faith that sanctifies are not two different kinds of faith. “Sanctify” simply means to make holy or to transform into Christlikeness. It is all by grace.
Therefore, it must also be through faith. For faith is the act of the soul that connects with grace, and receives it, and channels it as the power of obedience, and guards grace from being nullified through human boasting.
Paul makes this connection between faith and sanctification explicit in Galatians 2:20 (“I live by faith”). Sanctification is by the Spirit and by faith. Which is another way of saying that it is by grace and by faith. The Spirit is “the Spirit of grace” (Hebrews 10:29). God’s way of making us holy is by the Spirit; but the Spirit works through faith in the gospel.
The simple reason why the faith that justifies is also the faith that sanctifies is that both justification and sanctification are the work of sovereign grace. And it’s faith that corresponds to grace. Justification and sanctification are not the same kind of work (justification is the imputation of righteousness; sanctification is the impartation of righteousness), but they are both works of grace. Sanctification and justification are “grace upon grace” (John 1:16).
The human corollary of God’s free grace is faith. If both justification and sanctification are works of grace, it is natural that they would both be by faith.