In case we really are Christians, in case it is (Christianly) quite as it should be with "Christendom," a "Christian world" — then the New Testament is eo ipso no longer a guide for Christians, cannot be such
Under the assumed conditions, the New Testament neither is nor can be a guide for Christians — for the way is changed, is entirely different from the one in the New Testament.
The New Testament therefore, regarded as a guide for Christians, becomes, under the assumption we have made, a historical curiosity, pretty much like a guidebook to a particular country when everything in that country has been totally changed. Such a guidebook serves no longer the serious purpose of being useful to travelers in that country, but at the most it is worth reading for amusement. While one is making the journey easily by railway, one reads in the guidebook, "Here is Woolf's Gullet where one plunges 70,000 fathoms down under the earth"; while one sits and smokes one's cigar in the snug café, one reads in the guidebook, "Here it is a band of robbers has its stronghold, from which it issues to assault the travelers and maltreat them"; here it is, etc. Here it is; that is, here it was; for now (it is very amusing to imagine how it was), now there is no Woolf's Gullet but the railway, and no robber band, but a snug café.
If then we really are Christians, if it is quite as it should be with "Christendom" and a Christian world — then would I shout, loud enough if possible to be heard in heaven, "Thou infinite One, if in other respects Thou hast showed Thyself to be love, this verily was unloving on Thy part, that Thou hast not let men know that the New Testament is no longer a guide [guidebook] for Christians! How cruel, while all has been changed to the very opposite, and yet it is true that we are all Christians, to alarm the weak by the fact that Thy Word has not yet been repealed or altered!"
However this I cannot assume, that God could be like that; and therefore I am compelled to try another explanation, to which also I am much more inclined: this whole thing about "Christendom" and "a Christian world" is a knavish trick on man's part, the notion that we really are Christians is a vain conceit by force of the knavish trick; on the other hand, the New Testament, entirely unchanged, is the guidebook for Christians, for whom things will go in this world as one reads in the New Testament, and who should not let themselves be disturbed by the fact that for knavish Christians things go differently in this world, a knavish world.