Two weeks ago I threw out some thoughts about the unwieldy and oftentimes contested doctrine of sanctification. I said that the Reformed responsibility was to chart a course between antinomianism on the one hand, and moralism on the other. I think Reformed theology is uniquely positioned to fall into both of these errors if it's turned into an unbalanced system. For instance, an undue emphasis on the doctrine of the perseverance of the saints puts us in a position not unlike that of Tullian Tchividjian, and an undue emphasis on a sort of predestinarian pietism has a tendency to yield a yoke or uncertainty similar to what we would get if we collapsed justification into sanctification. So, I guess the question now is, "What does this middle path look like?
Like I mentioned two weeks ago, I think G.C. Berkouwer is out ahead of the pack in his proposal. It's not difficult to find a popular presentation of Reformed theology in which faith is the operative concept in reference to justification while effort is the operative concept in reference to sanctification. I wonder if the best option isn't to follow G.C. Berkouwer by advocating for an approach to the doctrine of sanctification which is just like the classic Reformed approach to justification: sanctification sola fide, or sanctification by faith alone.
The Reformed approach to the doctrine of sanctification should be no different than the Reformed approach to the doctrine of justification, for how could one be brought into the fold of God by the gift of faith, and while remaining a sinner, stay in the fold of God by anything other than the gift of faith? Thinking about the doctrine of sanctification in a sola fide framework is only consistent with a classically Reformed methodology in which God is always the primary actor in His relationship with humans, for it is He who reveals Himself to us first, and it is He who loves us before we could ever love Him (1 Jn. 4:19).
It is with this in mind that Richard Lints helpfully notes that, “there is no point…when Christ ceases to be the representative mediator between [the Christian] and God.” Indeed, because the believer remains a sinner (Rom. 7, 1 Jn. 1:8) even after conversion, it is always and will always be Christ who mediates the relationship between God and the Christian. There is no point in the Christian life during which the Christian will ever be able to stand on their own before the throne of God without the mediatorial work of Christ. Because this is the case, Berkouwer states that the nature of sanctification is such that the believer must always be in “constant ‘commerce’ with the forgiveness of sins and his continued dependence on it must—both in pastoral counselling and in dogmatic analysis—be laid bare, emphasized, and kept in sight.” Berkouwer goes on to formulate the relationship between an emphasis on grace and and emphasis on works in the process of sanctification in such a way that it is worth quoting at length:
“To the man who understands that a progressive sanctification must keep the windows of faith opened to the grace of God, the surprising multiformity of the Word of God will be intelligible. For one moment we are directed to follow after holiness and another to grow in the grace and knowledge of Jesus Christ. This multiformity preserves us both from passivity and from nomism. Any ‘striving,’ in this connection, receives its content from the fact of grace…The progress that is here meant is like the fruitbearing of branches in the vine. The branch, if broken from the vine, cannot bear fruit.”
Here, Berkouwer attempts to chart this middle path about which we've been thinking. He acknowledges the Scriptural imperative to grow in holiness while also acknowledging the Scriptural imperative to grow in knowledge of grace. Yet, at the same time he employs the Scriptural example (Jn. 15:5) of the vine and its branches to describe the Christian’s growth in holiness. Indeed, as the Holy Spirit unites the believer with Christ, the believer becomes the beneficiary of every one of Christ’s benefits, among which is sanctification through communion with him, and a “constant commerce” with repentance through faith in Christ.
Thus, Berkouwer has come completely full circle, for to be a vine attached to the branch of Christ is to constantly rely on Him, to find one’s source not only of life but of holiness as well in Him. This is a concept that, in a sense, seems to also account for Peterson’s possessive aspect of sanctification. Such a relationship between the Christian and Christ inevitably leads to a deeper “commerce” with repentance and grace, thus resulting in growth in holiness. Perhaps the most important element of Berkouwer’s exposition of sanctification is the fact that he does not ground the believer’s assurance in their growth in holiness, which is impossible to measure accurately due to both sin and finitude. This is an important pastoral note, for grounding assurance in justification rather than sanctification grounds assurance in the objective rather than subjective.
 Richard Lints, “Living by Faith—Alone?” in Sanctification: Explorations in Theology and Practice, ed. Kelly Kapic (Downers Grove, IL: InterVarsity Press, 2014), 43.
 G.C Berkouwer, Faith and Sanctification (Grand Rapids, MI: Wm. B. Eerdmans Publishing Co., 1952), 84.
 Berkouwer, Faith and Sanctification, 107.