The theory of divine government has always presented some difficulties to human reason, as we can see even in St. Augustine, who never clearly made up his mind whether the City of God was the actual church presided over by the Bishop of Rome or whether it was an ideal and invisible congregation of the saved.
But we may be sure that to plainer minds it was necessary to believe that God governs mankind through the agency of the visible church. The unsophisticated man may not be realistic, but he is literal; he would be quite incapable, we may be sure, of understanding what St. Thomas meant when he asked "why should not the same sacred letter … contain several senses founded on the literal?" He would accept all the senses but he would accept them all literally. And taking them literally he would have to believe that if God governs the world, he governs it, not in some obscure meaning of the term, but that he actually governs it, as a king who is mightier than Charlemagne, but not essentially unlike Charlemagne.
The disposition to believe in the rule of God depended, therefore, upon the capacity to believe in a visible church upon earth which holds its commission from God. In some form or another all simple people look to a priestly caste who make visible the divine power. Without some such actualization the human imagination falters and becomes vagrant. The Catholic Church by its splendor and its power and its universality during the Middle Ages must have made easily credible the conception of God the Ruler. It was a government exercising jurisdiction over the known world, powerful enough to depose princes, and at its head was the Pope who could prove by the evidence of scripture that he was the successor to Peter and was the Vice-gerent of God. To ask whether this grandiose claim was in fact true is, from the point of view of this argument, to miss the point. It was believed to be true in the Middle Ages. Because it was believed, the Church flourished. Because the Church flourished, it was ever so much easier to be certain that the claim was true. When men said that God ruled the world, they had evidence as convincing as we have when we say that the President is head of the United States Government; they were convinced because they came into daily contact with God's appointees administering God's laws.
It is this concrete sense of divine government which modern men have lost, and it may well be that this is where the Reformation has exercised its most revolutionary effect. What Luther did was to destroy the pretensions not only of the Roman Catholic Church, but of any church and of any priestly class to administer God's government on earth. The Protestant reformers may not have intended to destroy as deeply as they did; the theocracies established by Calvin and Knox imply as much. But, nevertheless, when Luther succeeded in defying the Holy See by rejecting its claim that it was the exclusive agent of God, he made it impossible for any other church to set up the same claim and sustain it for any length of time.
Now Christ says that not alone in the Church is there forgiveness of sins, but that where two or three are gathered together in His name, they shall have the right and the liberty to proclaim and promise to each other comfort and the forgiveness of sins … We are not only kings and the freest of all men, but also priests forever, a dignity far higher than kingship, because by that priesthood we are worthy to appear before God, to pray for others, and to teach one another mutually the things which are of God.
This denial of the special function of the priesthood did not, of course, originate with Luther. Its historical antecedents go back to the primitive Christians; there is quotable authority for it in St. Augustine. It was anticipated by Wyclif and Huss and by many of the mystics of the Middle Ages. But Luther, possibly because the times were ripe for it, translated the denial of the authority of the priesthood into a political revolution which divided Christendom. When the Reformation was an accomplished fact, men looked out upon the world and no longer saw a single Catholic Apostolic Church as the visible embodiment of God's government. A large part of mankind, and that an economically and politically powerful part, no longer believed that Christ gave to Simon Peter and his successors at the Roman See the Keys of the Kingdom of Heaven with the promise that "whatsoever thou shalt bind on earth shall be bound in heaven: and whatsoever thou shalt loose on earth shall be loosed in heaven."