Martin Luther always said that if the church is to be reformed, human beings cannot do it, for only God can reform his church. In order to think about this clearly, one must understand what 'church' means and what 'reform' means. One must also get clear on the impossibility that Luther finds that human beings have with respect to 'reforming the church'. I wish to explore these matters a bit below, seeking at all times to think through these themes from a perspective that is ruthlessly Lutheran.
'Church' means primarily, for Lutherans, the 'association of those with faith and Holy Spirit in the heart' (Apology, Article IV). As such this church is that to which the four Nicene predicates apply. 'One' is predicable of it because it is a set of those having the same attributes. 'Holy' is predicable of the church because those in which faith and the Holy Spirit reside may be called holy. 'Catholic' can be predicated of the church because it is everywhere, that is, there is no place where those with faith and the Holy Spirit are not. Finally, 'apostolic' can be predicated of the church because those with faith and Holy Spirit in the heart are those who have a successio fidei reaching back to apostolic times. This is the way that these predicates were standardly applied during Lutheran Orthodoxy.
The first thing to realize about construing 'church' in this way is that it seems not in need of being re-formed at all. The hidden church, what Melanchthon calls "the true church" is not the kind of thing that can be reformed, because if faith is not present in a person putatively in the church, the person is not actually in the church. The church has definite boundaries; it is a binary or digital phenomenon. There are no grades of this church; it simply is - - or is not.
So when Luther said that God is in the process of reforming his church, it may seem that he is not primarily interested in the hidden church, but rather the hidden church as it is revealed around Word and Sacrament. This church, of which the hidden church is a subset, is visible, and because of the presence of the hidden church within can be called by synechdoche 'church' as well. This visible church, which is not the true church, can be either purely formed or better formed. A felicitous forming would be one in which the gospel would regularly be preached in its purity and the sacrament rightly administered. An infelicitous ordering would be one in which it was difficult for the gospel to be proclaimed in its purity and the sacrament rightly administered. So the question is this: can this church only be reformed through divine causal power? Luther seems to say "yes".
But before we decide this easily in the affirmative, it is important to distinguish what powers human beings really have. In The Bondage of the Will, Luther decries any who, like Erasmus, would claim that human beings have a free will. But what is this claim to mean? What is meant by 'free will' and how does the presence or lack of free will relate to the issue of the reform of the church?
Luther does understand freedom of the will not primarily along the lines of being able to do other than what one did do, but more along the line of being able to do what one wants or rationally decides to do. Luther claims in the Bondage of the Will that one has no free will with respect to actualizing the desire or decision to be closer to God, to move toward God. While human beings are free with respect to "those things below them," they are not free with respect to those above them. A person can plant corn if they desire or rationally decide to do so. A person can build a house using asphalt or steel shingles. These things a person can do. What a person cannot do is move closer toward, or gain the favor or gifts of God; one cannot change the situation with respect to God by one's own efforts. The will is bound never so to properly relate to God. A human being cannot be his/her own reason or strenght believe in the Lord Jesus Christ or come to Him.
So how do these ruminations relate to the issue of the visible church? Luther says only God can reform it, not human beings. So what is the visible church, a thing above or a thing below human reason. Do human beings have power to affect the contour of the visible church?
The short answer to this must be that the visible church, in so far as it is an earthly institution, is truly a thing below human reason. As a thing below human reason, human beings can affect its structure such that felicitous attributes that get the gospel properly preached are either augmented or diminished. To reform the church in this way is something that human beings have the causal power to accomplish. One can either augment or diminish the tendency of the earthly insitution to proclaim purely the gospel and administer rightly the sacrament. Accordingly, human causal powers can affect the state of the visible church. Why is this? The answer is simple: The visible church, considered as an association of people gathered around Word and sacrament, is simply a temporal organization that either is properly ordered or imporoperly ordered to get the josb done: preach the gospel in its purity and administer the sacrament with propriety. So considered, human beings can affect the visible church.
However, human beings can make no headway in changing the breath of the hidden church. With respect to the divine, human beings are powerless. Human beings, no matter how properly they order the visible church for gospel purity, cannot increase nor decrease membership in set of all of those with faith and Holy Spirit in the heart. Only God can do this; only God can form again the hidden church he once brought into being. Just as human beings can do nothing to increase their own salvation, so can they do nothing to augment the boundaries of God's Church. Only God can do that.
So what have we learned? When Luther said that only God can reform his Church, he meant that only God can form again the boundaries of that hidden Church. The class of all those with faith and the Holy Spirit in the heart is homogenous. There exist no density points, no "closer thans" or "further froms" within the true church. Only God sets the boundaries of that church. But of the empirical church within which both the hidden church and "tares" reside, that church is a human, temporal organization and as such can be affected by the causal powers of human agents. Man and woman have freedom of the will with respect to the empirical church considered as a historical reality. They do not have freedom of the will with respect to the extension of the hidden church. The hidden church is a thing above human beings; the visibile church in which it resides is clearly a thing below human beings. Confusion abounds when these things are not clearly thought.
So against our first judgments we must hold that the church Luther is thinking only God can reform is the hidden church., and the church we see around us is fully susceptible for reform by exertion of human causal power and agency.