Bonhoeffer’s great insight in LPP as far as I can see, was to dimly perceive that, while religion was a predominant guise for Christianity throughout history this did not need to be the case – that Christianity could affirm all its central tenants without religion as he defined it (God could be affirmed without metaphysics – again he was not saying that this was ontologically better but rather was becoming historically nessesary). He saw religion as having served its time well, but which had finally reached its twilight.
This is interesting to me because I think it allows us to understand Fundamentalism in a different way. Namely, as an impotent reaction to the loss of religion. The attempt to place it back in the centre. Fundamentalism can thus be seen as the very evidence of the growing redundancy of religion. It is the violent kickback against the continual loss of ground that religion has had to concede in recent years. But for Bonhoeffer there is a way beyond an anemic religious Christianity that places God at the edge and a violent fundamentalism which impotently seeks to place religion in the centre and this is what he was hinting at.