Neither focuses on what I would call a philosophical theology; their subject is the Christian life. And their differences admittedly go further than France vs. Germany. They reflect important differences between Catholic and Protestant, and between the seventeenth and the twentieth centuries.
I had started to add that one was directed to an "ordinary" period, the other to a "crisis" period, but I'm not sure if that distinction entirely obtains. Nachfolge was written to Christians under the National Socialist regime, with dire consequences for anyone, of whatever conviction, who did not sit down, shut up, and do what he was told. The Introduction's intended audience wasn't under such pressures or temptations, but, appearing as it did between the end of the French Wars of Religion and the beginning of the Thirty Years War, its context is still one of intense conflict within Christendom about what it means to be a Christian.
Perhaps a better way of expressing the social difference is that, in the world of the Introduction, the Church is a functioning reality, even if there is bitter conflict between communions about some fundamental doctrinal and social assumptions. For Bonhoeffer, the Church itself is near collapse--not only the State-approved church, but even the Confessing Church, which he did so much to foster, is losing its way.
This loneliness of the individual for Bonhoeffer is not solely the result of the struggle with Nazism. There was always a strain of individualism in Lutheranism, the individual conviction brought on by the unmeetable demands of the Law, the denial of a "safe harbor" in the Church. Bonhoeffer knows Luther well. He also knows Kierkegaard: "Der Ruf Jesu in die Nachfolge macht den Jünger zum Einzelnen. Ob er will oder nicht, er muss sich entscheiden, er muss sich allein entscheiden. Es ist nicht eigene Wahl, Einzelner sein zu wollen, sondern Christus macht den Gerufenen zum Einzelnen. Jeder ist allein gerufen."
In this fifth chapter of part I of Nachfolge, Bonhoeffer addresses two of the most difficult passages of scripture, the saying of Jesus that his follower must hate father and mother, and the story of the sacrifice of Isaac. His reading of the Old Testament story follows closely that of Kierkegaard in his Fear and Trembling: Abraham's faith allows him to give up Isaac, and that very act is what allows Isaac to be returned to him. Bonhoeffer reads that narrative into the demand of Jesus to "hate" those closest--it is not, ultimately, demanded in order to really separate, but to emphasize that direct relationships can only be based on a renunciation and restoration. This is a hard, hard teaching, reminiscent of the old American folk song
You've got to walk that lonesome valley,
You've got to walk it for yourself.
Lord, nobody else can walk it for you,
You've got to walk it by yourself.
Kierkegaard's loneliness always strikes me as noble, principled, but almost pathological, because not called for by extraordinary circumstances. Bonhoeffer's is more understandable, since the path his conscience calls him to endangers anyone near him.
It is something of a relief to turn from the world of Bonhoeffer to that of St. François. The style of Bonhoeffer is strong, demanding. That of St. François is a light style, a sweet style, seeking as much to entice as to convict. "Le roi des abeilles ne se met point aux champs qu’il ne soit environné de tout son petit peuple, et la charité n’entre jamais dans un coeur qu’elle n’y loge avec soi le train des autres vertus, les exerçant et mettant en besogne ainsi qu’un capitaine fait ses soldats." This comparison of the "king" of bees, attended by his "petit peuple," with charity attended by the virtues, is typical of François' drawing on lovely images to convey his message.
But the greater distinction is that provided by the environment of the Church. The individual never faces the crisis alone. Devotion comes, not out of a radical confrontation and consequent forced decisiveness, but from listening to the voice of the Church, and, to a large extent, from following the various "methods" that the experience of the Church has bequeathed to her children: "...j’ai fait cette Introduction de cinq Parties, en la première desquelles je m’essaie, par quelques remontrances et exercices, de convertir le simple désir de Philothée en une entière résolution." The desire for devotion is assumed; by certain exercises that simple desire is allowed to blossom into a whole resolve.
This might seem mechanical, were it not, in fact, ultimately grounded in the love of God: "La vraie et vivante dévotion, o Philothée, présuppose l’amour de Dieu, ains elle n’est autre chose qu’un vrai amour de Dieu;... Bref, la dévotion n’est autre chose qu’une agilité et vivacité spirituelle par le moyen de laquelle la charité fait ses actions en nous, ou nous par elle, promptement et affectionnément; et comme il appartient à la charité de nous faire généralement et universellement pratiquer tous les commandements de Dieu, il appartient aussi à la dévotion de les nous faire faire promptement et diligemment." Devotion is what transforms the love of God in the soul to the activity of charity in the world, to that obedience to the commandments of God which is also, ultimately, the goal of Bonhoeffer's exhortations.
I do not presume to judge between the two. As a Catholic in a time of peace and relative prosperity I admittedly live more in the stable world presumed by St. François. Even so, I recognize that the Church is considerably more marginalized in contemporary society than in the seventeenth century, and that the relationship, even of devout Catholics, to our Church, is far different today--the practice of spiritual direction, even of participation in the sacrament of penance, is far from central to American Catholic life. We are largely as proudly individualistic (and lonely) as our Protestant counterparts.
This dissolution of Christendom--even if not so radical as the "religionless Christianity" expected by Bonhoeffer in his prison letters--tends to bring Bonhoeffer to renewed relevance. We are more and more brought to discipleship, not by catechesis and training, but by crisis. Not, thank God, from a crisis so severe as that which Bonhoeffer endured, but by the continual re-arising of serious, deadly evils, at home and abroad, which we are fully capable of ignoring and co-existing with in our comfortable little worlds. It is to that kind of complacency that Bonhoeffer speaks, equally to the Protestant and the Catholic.
In closing here I can't help noting the temporal coincidence of these two works with others I've been discussing in nearby posts. Nachfolge was published just two years before Finnegans Wake. The Introduction à la vie dévote was published between the appearance of the first and second parts of Don Quijote de la Mancha. There is one world, but many takes on it, and it is often hard to square them.