Richard Gaffin, in his Redemptive History and Biblical Interpretation, explains Christian sanctification as eschatological life reaching the now. Just as justification is the end judgement reaching the now, so sanctification is the end reaching the now.
The Reformation… was a (re)discovery, at least implicitly, of the eschatological heart of the gospel: the sola gratia principle is eschatological in essence. Justification by faith, as the Reformers came to understand and experience it, is an anticipation of the final judgement. It means that a favourable verdict at the last judgement is not an anxious, uncertain hope, but a present possession, the confident and stable basis of the Christian life. Romans 8:1, which they clung to, is a decidedly eschatological pronouncement.
However while the Reformation and its children have grasped, at least intuitively, the eschatological thrust of the gospel for justification, that is not nearly the case for sanctification and the work of the Spirit. Undeniable is a tendency, at least in practice, to separate or even polarize justification and sanctification. Justification, on the one hand, is seen as what God does, once for all and perfectly: sanctification, on the other hand, is what the believer does, imperfectly. Sanctification is viewed as the response of the believer, an expression of gratitude from our side for salvation defined in terms of justification and the forgiveness of sins — usually with an emphasis on the inadequate and even impoverished quality of the gratitude expressed.
The intention of such an emphasis is no doubt to safeguard the totally gratuitous character of justification. But church history has made all too evident that the apparently inevitable outcome of such an emphasis is the rise of moralism, the reintroduction into the Christian life of a refined works-principle, more or less divorced from the faith that justifies and eventually leaving no room for faith. What is resolutely rejected at the front door of justification comes in through the back door of sanctification and takes over the whole house.
Certainly we must be on safeguard against all notions of sinless perfection. Forms of “entire” sanctification or “higher”, “victorious” life, supposedly achieved by a distinct act of faith subsequent to justification, operate with domesticated, voluntaristic notions of sin and invariably de-eschatologize the gospel and in their own way, despite their intention, end up promoting moralism. We must not forget that “in this life even the holiest have only a small beginning” (HC, answer 114).
But — and this is the point — the beginning, however small, is an eschatological beginning. It stands under the apostolic promise that “He who began a good work in you will perfect it until the day of Jesus Christ (Phil 1:6, NASB). Sanctification, no less than justification, is God’s work. In the NT there is no more basic perspective on sanctification and renewal than that expressed in Romans 6: It is a continual “living to God” (v. 11) of those who are “alive from the dead” (v. 13). Elsewhere, it is a matter of the “good works” of the eschatological new creation, for which the church has already been “created in Christ Jesus” (Eph 2:10). In their sanctification, believers begin at the “top”, because they begin with Christ; in him they are those who are “perfect” (1 Cor 2:6) and “spiritual” (v. 15), even when they have to be admonished as “carnal” (3:1, 3).
I think this is right. The gospel, as the cross shows, brings the last judgement and the kingdom to the “here and now”. And so, what is sanctification, but an eschatological realization of the here and now of kingdom life. We are called to live in the here and now how we will live in our future, glorified state.
And thus, sanctification, is an ever-increasing practical realization, apprehension, and outworking of the life which will be given in full at the last day.