“I ask whether it is wise to speak of faith as the one thing needful, and the only thing required, as many seem to do nowadays in handling the doctrine of sanctification. Is it wise to proclaim in so bald, naked, and unqualified a way as many do that the holiness of converted people is by faith only, and not at all by personal exertion? Is it according to the proportion of God’s Word? I doubt it.
That faith in Christ is the root of all holiness; that the first step towards a holy life is to believe on Christ; that until we believe we have not a jot of holiness; that union with Christ by faith is the secret of both beginning to be holy and continuing holy; that the life that we live in the flesh, we must live by faith in the Son of God; that faith purifies the heart; that faith is the victory which overcomes the world; that by faith the elders obtained a good report—all these are truths which no well instructed Christian will ever think of denying. But surely the Scriptures teach us that in following holiness the true Christian needs personal exertion and work as well as faith. The very same apostle who says in one place, “the life that I live in the flesh I live by the faith of the Son of God,’ says in another place, “I Fight,” “I run,” “I keep under my body”; and in other places, “let us cleanse ourselves,” “Let us labour,” “Let us lay aside every weight.”
Moreover, the Scriptures nowhere teach us that faith sanctifies us in the same sense and in the same manner that faith justifies us! Justifying faith is a grace that “worketh not,’ but simply trusts, rests, and leans on Christ (Rom 4:5). Sanctifying faith is a grace of which the very life is action: it worketh by love,” and, like a mainspring, moves the whole inward man” (Gal 5:6)…
Without controversy, in the matter of our justification before God, faith in Christ is the one thing needful. All that simply believe are justified. Righteousness is imputed “to him that worketh not but believeth” (Rom 4:5). It is thoroughly scriptural and right to say, “Faith alone justifies.” But it is not equally scriptural and right to say, “Faith alone sanctifies.” The saying requires very large qualification. Let one fact suffice. We are frequently told that a man is “justified by faith without the deeds of the law” by St. Paul. But not once are we told that we are “sanctified by faith without the deeds of the law.”
I once wrote a post entitled "Sanctification by faith." I stand by what I said. I think that sanctification flows from union with Christ. Christ's grace in us imparts and empowers sanctification. I would also not want what I wrote there to be misconstrued as a denial of real effort in sanctification. We are to work out our salvation. We are to mortify the flesh. The only way to do this is with and through the Holy Spirit and Christ's power operative in us.
There are two sides to avoid here on sanctification.
On the one hand, we should not have a casual 'let go and let God' where we are not actively fighting sin, seeking God's faith, seeking the empowerment of his grace, and even working at true self-control.
On the other hand, we should not think that sanctification is all up to me. We should not in our efforts of sanctification develop a self-righteous spirit. It is God who works in me to bring to completion the good work that he has begun.